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7 Network Working Group J. Klensin
8 Request for Comments: 5321 October 2008
9 Obsoletes: 2821
10 Updates: 1123
11 Category: Standards Track
14 Simple Mail Transfer Protocol
16 Status of This Memo
18 This document specifies an Internet standards track protocol for the
19 Internet community, and requests discussion and suggestions for
20 improvements. Please refer to the current edition of the "Internet
21 Official Protocol Standards" (STD 1) for the standardization state
22 and status of this protocol. Distribution of this memo is unlimited.
24 Abstract
26 This document is a specification of the basic protocol for Internet
27 electronic mail transport. It consolidates, updates, and clarifies
28 several previous documents, making all or parts of most of them
29 obsolete. It covers the SMTP extension mechanisms and best practices
30 for the contemporary Internet, but does not provide details about
31 particular extensions. Although SMTP was designed as a mail
32 transport and delivery protocol, this specification also contains
33 information that is important to its use as a "mail submission"
34 protocol for "split-UA" (User Agent) mail reading systems and mobile
35 environments.
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63 Table of Contents
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
66 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . . 5
67 1.2. History and Context for This Document . . . . . . . . . . 5
68 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6
69 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . . 7
70 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7
71 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9
72 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . . 9
73 2.2.2. Definition and Registration of Extensions . . . . . . 10
74 2.2.3. Special Issues with Extensions . . . . . . . . . . . . 11
75 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11
76 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . . 11
77 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12
78 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . . 12
79 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . 13
80 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . . 13
81 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . . 14
82 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . . 14
83 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 14
84 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15
85 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15
86 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 15
87 2.4. General Syntax Principles and Transaction Model . . . . . 16
88 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17
89 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 18
90 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 18
91 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19
92 3.4. Forwarding for Address Correction or Updating . . . . . . 21
93 3.5. Commands for Debugging Addresses . . . . . . . . . . . . . 22
94 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 22
95 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . . 24
96 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . . 25
97 3.5.4. Semantics and Applications of EXPN . . . . . . . . . . 26
98 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26
99 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . . 26
100 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . . 26
101 3.6.3. Message Submission Servers as Relays . . . . . . . . . 27
102 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 28
103 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 28
104 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . . 29
105 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 29
106 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 29
107 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30
108 3.8. Terminating Sessions and Connections . . . . . . . . . . . 30
109 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31
110 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 31
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119 3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . . 31
120 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 32
121 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 32
122 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . . 32
123 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 41
124 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . . 43
125 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 44
126 4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . . 46
127 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . . 46
128 4.2.1. Reply Code Severities and Theory . . . . . . . . . . . 48
129 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . . 50
130 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . . 52
131 4.2.4. Reply Code 502 . . . . . . . . . . . . . . . . . . . . 53
132 4.2.5. Reply Codes after DATA and the Subsequent
133 <CRLF>.<CRLF> . . . . . . . . . . . . . . . . . . . . 53
134 4.3. Sequencing of Commands and Replies . . . . . . . . . . . . 54
135 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 54
136 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 55
137 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 57
138 4.5. Additional Implementation Issues . . . . . . . . . . . . . 61
139 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . . 61
140 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . . 62
141 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . . 62
142 4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . . 62
143 4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . . 63
144 4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . . 63
145 4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . . 63
146 4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . . 63
147 4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . . 63
148 4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 63
149 4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 63
150 4.5.3.1.8. Recipients Buffer . . . . . . . . . . . . . . 64
151 4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . . 64
152 4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . . 64
153 4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . . 65
154 4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . . 65
155 4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 65
156 4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 65
157 4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . . 66
158 4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 66
159 4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 66
160 4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . . 66
161 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . . 66
162 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 68
163 5. Address Resolution and Mail Handling . . . . . . . . . . . . . 69
164 5.1. Locating the Target Host . . . . . . . . . . . . . . . . . 69
165 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 71
166 6. Problem Detection and Handling . . . . . . . . . . . . . . . . 71
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175 6.1. Reliable Delivery and Replies by Email . . . . . . . . . . 71
176 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . . 72
177 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 73
178 6.4. Compensating for Irregularities . . . . . . . . . . . . . 73
179 7. Security Considerations . . . . . . . . . . . . . . . . . . . 75
180 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . . 75
181 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . . 76
182 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . . 76
183 7.4. Mail Rerouting Based on the 251 and 551 Response Codes . . 77
184 7.5. Information Disclosure in Announcements . . . . . . . . . 77
185 7.6. Information Disclosure in Trace Fields . . . . . . . . . . 78
186 7.7. Information Disclosure in Message Forwarding . . . . . . . 78
187 7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 78
188 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . . 78
189 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 79
190 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 80
191 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 81
192 10.1. Normative References . . . . . . . . . . . . . . . . . . . 81
193 10.2. Informative References . . . . . . . . . . . . . . . . . . 82
194 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . . 85
195 Appendix B. Generating SMTP Commands from RFC 822 Header
196 Fields . . . . . . . . . . . . . . . . . . . . . . . 85
197 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . . 86
198 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . . 87
199 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 88
200 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 89
201 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 90
202 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . . 92
203 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 92
204 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 93
205 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
206 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . . 93
207 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
208 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . . 94
209 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 94
210 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . . 94
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231 1. Introduction
233 1.1. Transport of Electronic Mail
235 The objective of the Simple Mail Transfer Protocol (SMTP) is to
236 transfer mail reliably and efficiently.
238 SMTP is independent of the particular transmission subsystem and
239 requires only a reliable ordered data stream channel. While this
240 document specifically discusses transport over TCP, other transports
241 are possible. Appendices to RFC 821 [1] describe some of them.
243 An important feature of SMTP is its capability to transport mail
244 across multiple networks, usually referred to as "SMTP mail relaying"
245 (see Section 3.6). A network consists of the mutually-TCP-accessible
246 hosts on the public Internet, the mutually-TCP-accessible hosts on a
247 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
248 environment utilizing a non-TCP transport-level protocol. Using
249 SMTP, a process can transfer mail to another process on the same
250 network or to some other network via a relay or gateway process
251 accessible to both networks.
253 In this way, a mail message may pass through a number of intermediate
254 relay or gateway hosts on its path from sender to ultimate recipient.
255 The Mail eXchanger mechanisms of the domain name system (RFC 1035
256 [2], RFC 974 [12], and Section 5 of this document) are used to
257 identify the appropriate next-hop destination for a message being
258 transported.
260 1.2. History and Context for This Document
262 This document is a specification of the basic protocol for the
263 Internet electronic mail transport. It consolidates, updates and
264 clarifies, but does not add new or change existing functionality of
265 the following:
267 o the original SMTP (Simple Mail Transfer Protocol) specification of
268 RFC 821 [1],
270 o domain name system requirements and implications for mail
271 transport from RFC 1035 [2] and RFC 974 [12],
273 o the clarifications and applicability statements in RFC 1123 [3],
274 and
276 o material drawn from the SMTP Extension mechanisms in RFC 1869
277 [13].
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287 o Editorial and clarification changes to RFC 2821 [14] to bring that
288 specification to Draft Standard.
290 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC
291 1123 (replacing the mail transport materials of RFC 1123). However,
292 RFC 821 specifies some features that were not in significant use in
293 the Internet by the mid-1990s and (in appendices) some additional
294 transport models. Those sections are omitted here in the interest of
295 clarity and brevity; readers needing them should refer to RFC 821.
297 It also includes some additional material from RFC 1123 that required
298 amplification. This material has been identified in multiple ways,
299 mostly by tracking flaming on various lists and newsgroups and
300 problems of unusual readings or interpretations that have appeared as
301 the SMTP extensions have been deployed. Where this specification
302 moves beyond consolidation and actually differs from earlier
303 documents, it supersedes them technically as well as textually.
305 Although SMTP was designed as a mail transport and delivery protocol,
306 this specification also contains information that is important to its
307 use as a "mail submission" protocol, as recommended for Post Office
308 Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
309 [17]). In general, the separate mail submission protocol specified
310 in RFC 4409 [18] is now preferred to direct use of SMTP; more
311 discussion of that subject appears in that document.
313 Section 2.3 provides definitions of terms specific to this document.
314 Except when the historical terminology is necessary for clarity, this
315 document uses the current 'client' and 'server' terminology to
316 identify the sending and receiving SMTP processes, respectively.
318 A companion document, RFC 5322 [4], discusses message header sections
319 and bodies and specifies formats and structures for them.
321 1.3. Document Conventions
323 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
324 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
325 document are to be interpreted as described in RFC 2119 [5]. As each
326 of these terms was intentionally and carefully chosen to improve the
327 interoperability of email, each use of these terms is to be treated
328 as a conformance requirement.
330 Because this document has a long history and to avoid the risk of
331 various errors and of confusing readers and documents that point to
332 this one, most examples and the domain names they contain are
333 preserved from RFC 2821. Readers are cautioned that these are
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343 illustrative examples that should not actually be used in either code
344 or configuration files.
346 2. The SMTP Model
348 2.1. Basic Structure
350 The SMTP design can be pictured as:
352 +----------+ +----------+
353 +------+ | | | |
354 | User |<-->| | SMTP | |
355 +------+ | Client- |Commands/Replies| Server- |
356 +------+ | SMTP |<-------------->| SMTP | +------+
357 | File |<-->| | and Mail | |<-->| File |
358 |System| | | | | |System|
359 +------+ +----------+ +----------+ +------+
360 SMTP client SMTP server
362 When an SMTP client has a message to transmit, it establishes a two-
363 way transmission channel to an SMTP server. The responsibility of an
364 SMTP client is to transfer mail messages to one or more SMTP servers,
365 or report its failure to do so.
367 The means by which a mail message is presented to an SMTP client, and
368 how that client determines the identifier(s) ("names") of the
369 domain(s) to which mail messages are to be transferred, is a local
370 matter, and is not addressed by this document. In some cases, the
371 designated domain(s), or those determined by an SMTP client, will
372 identify the final destination(s) of the mail message. In other
373 cases, common with SMTP clients associated with implementations of
374 the POP (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
375 protocols, or when the SMTP client is inside an isolated transport
376 service environment, the domain determined will identify an
377 intermediate destination through which all mail messages are to be
378 relayed. SMTP clients that transfer all traffic regardless of the
379 target domains associated with the individual messages, or that do
380 not maintain queues for retrying message transmissions that initially
381 cannot be completed, may otherwise conform to this specification but
382 are not considered fully-capable. Fully-capable SMTP
383 implementations, including the relays used by these less capable
384 ones, and their destinations, are expected to support all of the
385 queuing, retrying, and alternate address functions discussed in this
386 specification. In many situations and configurations, the less-
387 capable clients discussed above SHOULD be using the message
388 submission protocol (RFC 4409 [18]) rather than SMTP.
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399 The means by which an SMTP client, once it has determined a target
400 domain, determines the identity of an SMTP server to which a copy of
401 a message is to be transferred, and then performs that transfer, is
402 covered by this document. To effect a mail transfer to an SMTP
403 server, an SMTP client establishes a two-way transmission channel to
404 that SMTP server. An SMTP client determines the address of an
405 appropriate host running an SMTP server by resolving a destination
406 domain name to either an intermediate Mail eXchanger host or a final
407 target host.
409 An SMTP server may be either the ultimate destination or an
410 intermediate "relay" (that is, it may assume the role of an SMTP
411 client after receiving the message) or "gateway" (that is, it may
412 transport the message further using some protocol other than SMTP).
413 SMTP commands are generated by the SMTP client and sent to the SMTP
414 server. SMTP replies are sent from the SMTP server to the SMTP
415 client in response to the commands.
417 In other words, message transfer can occur in a single connection
418 between the original SMTP-sender and the final SMTP-recipient, or can
419 occur in a series of hops through intermediary systems. In either
420 case, once the server has issued a success response at the end of the
421 mail data, a formal handoff of responsibility for the message occurs:
422 the protocol requires that a server MUST accept responsibility for
423 either delivering the message or properly reporting the failure to do
424 so (see Sections 6.1, 6.2, and 7.8, below).
426 Once the transmission channel is established and initial handshaking
427 is completed, the SMTP client normally initiates a mail transaction.
428 Such a transaction consists of a series of commands to specify the
429 originator and destination of the mail and transmission of the
430 message content (including any lines in the header section or other
431 structure) itself. When the same message is sent to multiple
432 recipients, this protocol encourages the transmission of only one
433 copy of the data for all recipients at the same destination (or
434 intermediate relay) host.
436 The server responds to each command with a reply; replies may
437 indicate that the command was accepted, that additional commands are
438 expected, or that a temporary or permanent error condition exists.
439 Commands specifying the sender or recipients may include server-
440 permitted SMTP service extension requests, as discussed in
441 Section 2.2. The dialog is purposely lock-step, one-at-a-time,
442 although this can be modified by mutually agreed upon extension
443 requests such as command pipelining (RFC 2920 [19]).
445 Once a given mail message has been transmitted, the client may either
446 request that the connection be shut down or may initiate other mail
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455 transactions. In addition, an SMTP client may use a connection to an
456 SMTP server for ancillary services such as verification of email
457 addresses or retrieval of mailing list subscriber addresses.
459 As suggested above, this protocol provides mechanisms for the
460 transmission of mail. Historically, this transmission normally
461 occurred directly from the sending user's host to the receiving
462 user's host when the two hosts are connected to the same transport
463 service. When they are not connected to the same transport service,
464 transmission occurs via one or more relay SMTP servers. A very
465 common case in the Internet today involves submission of the original
466 message to an intermediate, "message submission" server, which is
467 similar to a relay but has some additional properties; such servers
468 are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
469 An intermediate host that acts as either an SMTP relay or as a
470 gateway into some other transmission environment is usually selected
471 through the use of the domain name service (DNS) Mail eXchanger
472 mechanism.
474 Usually, intermediate hosts are determined via the DNS MX record, not
475 by explicit "source" routing (see Section 5 and Appendix C and
476 Appendix F.2).
478 2.2. The Extension Model
480 2.2.1. Background
482 In an effort that started in 1990, approximately a decade after RFC
483 821 was completed, the protocol was modified with a "service
484 extensions" model that permits the client and server to agree to
485 utilize shared functionality beyond the original SMTP requirements.
486 The SMTP extension mechanism defines a means whereby an extended SMTP
487 client and server may recognize each other, and the server can inform
488 the client as to the service extensions that it supports.
490 Contemporary SMTP implementations MUST support the basic extension
491 mechanisms. For instance, servers MUST support the EHLO command even
492 if they do not implement any specific extensions and clients SHOULD
493 preferentially utilize EHLO rather than HELO. (However, for
494 compatibility with older conforming implementations, SMTP clients and
495 servers MUST support the original HELO mechanisms as a fallback.)
496 Unless the different characteristics of HELO must be identified for
497 interoperability purposes, this document discusses only EHLO.
499 SMTP is widely deployed and high-quality implementations have proven
500 to be very robust. However, the Internet community now considers
501 some services to be important that were not anticipated when the
502 protocol was first designed. If support for those services is to be
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511 added, it must be done in a way that permits older implementations to
512 continue working acceptably. The extension framework consists of:
514 o The SMTP command EHLO, superseding the earlier HELO,
516 o a registry of SMTP service extensions,
518 o additional parameters to the SMTP MAIL and RCPT commands, and
520 o optional replacements for commands defined in this protocol, such
521 as for DATA in non-ASCII transmissions (RFC 3030 [20]).
523 SMTP's strength comes primarily from its simplicity. Experience with
524 many protocols has shown that protocols with few options tend towards
525 ubiquity, whereas protocols with many options tend towards obscurity.
527 Each and every extension, regardless of its benefits, must be
528 carefully scrutinized with respect to its implementation, deployment,
529 and interoperability costs. In many cases, the cost of extending the
530 SMTP service will likely outweigh the benefit.
532 2.2.2. Definition and Registration of Extensions
534 The IANA maintains a registry of SMTP service extensions. A
535 corresponding EHLO keyword value is associated with each extension.
536 Each service extension registered with the IANA must be defined in a
537 formal Standards-Track or IESG-approved Experimental protocol
538 document. The definition must include:
540 o the textual name of the SMTP service extension;
542 o the EHLO keyword value associated with the extension;
544 o the syntax and possible values of parameters associated with the
545 EHLO keyword value;
547 o any additional SMTP verbs associated with the extension
548 (additional verbs will usually be, but are not required to be, the
549 same as the EHLO keyword value);
551 o any new parameters the extension associates with the MAIL or RCPT
552 verbs;
554 o a description of how support for the extension affects the
555 behavior of a server and client SMTP; and
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567 o the increment by which the extension is increasing the maximum
568 length of the commands MAIL and/or RCPT, over that specified in
569 this Standard.
571 In addition, any EHLO keyword value starting with an upper or lower
572 case "X" refers to a local SMTP service extension used exclusively
573 through bilateral agreement. Keywords beginning with "X" MUST NOT be
574 used in a registered service extension. Conversely, keyword values
575 presented in the EHLO response that do not begin with "X" MUST
576 correspond to a Standard, Standards-Track, or IESG-approved
577 Experimental SMTP service extension registered with IANA. A
578 conforming server MUST NOT offer non-"X"-prefixed keyword values that
579 are not described in a registered extension.
581 Additional verbs and parameter names are bound by the same rules as
582 EHLO keywords; specifically, verbs beginning with "X" are local
583 extensions that may not be registered or standardized. Conversely,
584 verbs not beginning with "X" must always be registered.
586 2.2.3. Special Issues with Extensions
588 Extensions that change fairly basic properties of SMTP operation are
589 permitted. The text in other sections of this document must be
590 understood in that context. In particular, extensions can change the
591 minimum limits specified in Section 4.5.3, can change the ASCII
592 character set requirement as mentioned above, or can introduce some
593 optional modes of message handling.
595 In particular, if an extension implies that the delivery path
596 normally supports special features of that extension, and an
597 intermediate SMTP system finds a next hop that does not support the
598 required extension, it MAY choose, based on the specific extension
599 and circumstances, to requeue the message and try later and/or try an
600 alternate MX host. If this strategy is employed, the timeout to fall
601 back to an unextended format (if one is available) SHOULD be less
602 than the normal timeout for bouncing as undeliverable (e.g., if
603 normal timeout is three days, the requeue timeout before attempting
604 to transmit the mail without the extension might be one day).
606 2.3. SMTP Terminology
608 2.3.1. Mail Objects
610 SMTP transports a mail object. A mail object contains an envelope
611 and content.
613 The SMTP envelope is sent as a series of SMTP protocol units
614 (described in Section 3). It consists of an originator address (to
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623 which error reports should be directed), one or more recipient
624 addresses, and optional protocol extension material. Historically,
625 variations on the reverse-path (originator) address specification
626 command (MAIL) could be used to specify alternate delivery modes,
627 such as immediate display; those variations have now been deprecated
628 (see Appendix F and Appendix F.6).
630 The SMTP content is sent in the SMTP DATA protocol unit and has two
631 parts: the header section and the body. If the content conforms to
632 other contemporary standards, the header section consists of a
633 collection of header fields, each consisting of a header name, a
634 colon, and data, structured as in the message format specification
635 (RFC 5322 [4]); the body, if structured, is defined according to MIME
636 (RFC 2045 [21]). The content is textual in nature, expressed using
637 the US-ASCII repertoire [6]. Although SMTP extensions (such as
638 "8BITMIME", RFC 1652 [22]) may relax this restriction for the content
639 body, the content header fields are always encoded using the US-ASCII
640 repertoire. Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
641 define an algorithm for representing header values outside the US-
642 ASCII repertoire, while still encoding them using the US-ASCII
643 repertoire.
645 2.3.2. Senders and Receivers
647 In RFC 821, the two hosts participating in an SMTP transaction were
648 described as the "SMTP-sender" and "SMTP-receiver". This document
649 has been changed to reflect current industry terminology and hence
650 refers to them as the "SMTP client" (or sometimes just "the client")
651 and "SMTP server" (or just "the server"), respectively. Since a
652 given host may act both as server and client in a relay situation,
653 "receiver" and "sender" terminology is still used where needed for
654 clarity.
656 2.3.3. Mail Agents and Message Stores
658 Additional mail system terminology became common after RFC 821 was
659 published and, where convenient, is used in this specification. In
660 particular, SMTP servers and clients provide a mail transport service
661 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User
662 Agents" (MUAs or UAs) are normally thought of as the sources and
663 targets of mail. At the source, an MUA might collect mail to be
664 transmitted from a user and hand it off to an MTA; the final
665 ("delivery") MTA would be thought of as handing the mail off to an
666 MUA (or at least transferring responsibility to it, e.g., by
667 depositing the message in a "message store"). However, while these
668 terms are used with at least the appearance of great precision in
669 other environments, the implied boundaries between MUAs and MTAs
670 often do not accurately match common, and conforming, practices with
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679 Internet mail. Hence, the reader should be cautious about inferring
680 the strong relationships and responsibilities that might be implied
681 if these terms were used elsewhere.
683 2.3.4. Host
685 For the purposes of this specification, a host is a computer system
686 attached to the Internet (or, in some cases, to a private TCP/IP
687 network) and supporting the SMTP protocol. Hosts are known by names
688 (see the next section); they SHOULD NOT be identified by numerical
689 addresses, i.e., by address literals as described in Section 4.1.2.
691 2.3.5. Domain Names
693 A domain name (or often just a "domain") consists of one or more
694 components, separated by dots if more than one appears. In the case
695 of a top-level domain used by itself in an email address, a single
696 string is used without any dots. This makes the requirement,
697 described in more detail below, that only fully-qualified domain
698 names appear in SMTP transactions on the public Internet,
699 particularly important where top-level domains are involved. These
700 components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
701 for SMTP purposes to consist of a sequence of letters, digits, and
702 hyphens drawn from the ASCII character set [6]. Domain names are
703 used as names of hosts and of other entities in the domain name
704 hierarchy. For example, a domain may refer to an alias (label of a
705 CNAME RR) or the label of Mail eXchanger records to be used to
706 deliver mail instead of representing a host name. See RFC 1035 [2]
707 and Section 5 of this specification.
709 The domain name, as described in this document and in RFC 1035 [2],
710 is the entire, fully-qualified name (often referred to as an "FQDN").
711 A domain name that is not in FQDN form is no more than a local alias.
712 Local aliases MUST NOT appear in any SMTP transaction.
714 Only resolvable, fully-qualified domain names (FQDNs) are permitted
715 when domain names are used in SMTP. In other words, names that can
716 be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
717 in Section 5) are permitted, as are CNAME RRs whose targets can be
718 resolved, in turn, to MX or address RRs. Local nicknames or
719 unqualified names MUST NOT be used. There are two exceptions to the
720 rule requiring FQDNs:
722 o The domain name given in the EHLO command MUST be either a primary
723 host name (a domain name that resolves to an address RR) or, if
724 the host has no name, an address literal, as described in
725 Section 4.1.3 and discussed further in the EHLO discussion of
726 Section 4.1.4.
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735 o The reserved mailbox name "postmaster" may be used in a RCPT
736 command without domain qualification (see Section 4.1.1.3) and
737 MUST be accepted if so used.
739 2.3.6. Buffer and State Table
741 SMTP sessions are stateful, with both parties carefully maintaining a
742 common view of the current state. In this document, we model this
743 state by a virtual "buffer" and a "state table" on the server that
744 may be used by the client to, for example, "clear the buffer" or
745 "reset the state table", causing the information in the buffer to be
746 discarded and the state to be returned to some previous state.
748 2.3.7. Commands and Replies
750 SMTP commands and, unless altered by a service extension, message
751 data, are transmitted from the sender to the receiver via the
752 transmission channel in "lines".
754 An SMTP reply is an acknowledgment (positive or negative) sent in
755 "lines" from receiver to sender via the transmission channel in
756 response to a command. The general form of a reply is a numeric
757 completion code (indicating failure or success) usually followed by a
758 text string. The codes are for use by programs and the text is
759 usually intended for human users. RFC 3463 [25], specifies further
760 structuring of the reply strings, including the use of supplemental
761 and more specific completion codes (see also RFC 5248 [26]).
763 2.3.8. Lines
765 Lines consist of zero or more data characters terminated by the
766 sequence ASCII character "CR" (hex value 0D) followed immediately by
767 ASCII character "LF" (hex value 0A). This termination sequence is
768 denoted as <CRLF> in this document. Conforming implementations MUST
769 NOT recognize or generate any other character or character sequence
770 as a line terminator. Limits MAY be imposed on line lengths by
771 servers (see Section 4).
773 In addition, the appearance of "bare" "CR" or "LF" characters in text
774 (i.e., either without the other) has a long history of causing
775 problems in mail implementations and applications that use the mail
776 system as a tool. SMTP client implementations MUST NOT transmit
777 these characters except when they are intended as line terminators
778 and then MUST, as indicated above, transmit them only as a <CRLF>
779 sequence.
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791 2.3.9. Message Content and Mail Data
793 The terms "message content" and "mail data" are used interchangeably
794 in this document to describe the material transmitted after the DATA
795 command is accepted and before the end of data indication is
796 transmitted. Message content includes the message header section and
797 the possibly structured message body. The MIME specification (RFC
798 2045 [21]) provides the standard mechanisms for structured message
799 bodies.
801 2.3.10. Originator, Delivery, Relay, and Gateway Systems
803 This specification makes a distinction among four types of SMTP
804 systems, based on the role those systems play in transmitting
805 electronic mail. An "originating" system (sometimes called an SMTP
806 originator) introduces mail into the Internet or, more generally,
807 into a transport service environment. A "delivery" SMTP system is
808 one that receives mail from a transport service environment and
809 passes it to a mail user agent or deposits it in a message store that
810 a mail user agent is expected to subsequently access. A "relay" SMTP
811 system (usually referred to just as a "relay") receives mail from an
812 SMTP client and transmits it, without modification to the message
813 data other than adding trace information, to another SMTP server for
814 further relaying or for delivery.
816 A "gateway" SMTP system (usually referred to just as a "gateway")
817 receives mail from a client system in one transport environment and
818 transmits it to a server system in another transport environment.
819 Differences in protocols or message semantics between the transport
820 environments on either side of a gateway may require that the gateway
821 system perform transformations to the message that are not permitted
822 to SMTP relay systems. For the purposes of this specification,
823 firewalls that rewrite addresses should be considered as gateways,
824 even if SMTP is used on both sides of them (see RFC 2979 [27]).
826 2.3.11. Mailbox and Address
828 As used in this specification, an "address" is a character string
829 that identifies a user to whom mail will be sent or a location into
830 which mail will be deposited. The term "mailbox" refers to that
831 depository. The two terms are typically used interchangeably unless
832 the distinction between the location in which mail is placed (the
833 mailbox) and a reference to it (the address) is important. An
834 address normally consists of user and domain specifications. The
835 standard mailbox naming convention is defined to be
836 "local-part@domain"; contemporary usage permits a much broader set of
837 applications than simple "user names". Consequently, and due to a
838 long history of problems when intermediate hosts have attempted to
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847 optimize transport by modifying them, the local-part MUST be
848 interpreted and assigned semantics only by the host specified in the
849 domain part of the address.
851 2.4. General Syntax Principles and Transaction Model
853 SMTP commands and replies have a rigid syntax. All commands begin
854 with a command verb. All replies begin with a three digit numeric
855 code. In some commands and replies, arguments are required following
856 the verb or reply code. Some commands do not accept arguments (after
857 the verb), and some reply codes are followed, sometimes optionally,
858 by free form text. In both cases, where text appears, it is
859 separated from the verb or reply code by a space character. Complete
860 definitions of commands and replies appear in Section 4.
862 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
863 and extension name keywords) are not case sensitive, with the sole
864 exception in this specification of a mailbox local-part (SMTP
865 Extensions may explicitly specify case-sensitive elements). That is,
866 a command verb, an argument value other than a mailbox local-part,
867 and free form text MAY be encoded in upper case, lower case, or any
868 mixture of upper and lower case with no impact on its meaning. The
869 local-part of a mailbox MUST BE treated as case sensitive.
870 Therefore, SMTP implementations MUST take care to preserve the case
871 of mailbox local-parts. In particular, for some hosts, the user
872 "smith" is different from the user "Smith". However, exploiting the
873 case sensitivity of mailbox local-parts impedes interoperability and
874 is discouraged. Mailbox domains follow normal DNS rules and are
875 hence not case sensitive.
877 A few SMTP servers, in violation of this specification (and RFC 821)
878 require that command verbs be encoded by clients in upper case.
879 Implementations MAY wish to employ this encoding to accommodate those
880 servers.
882 The argument clause consists of a variable-length character string
883 ending with the end of the line, i.e., with the character sequence
884 <CRLF>. The receiver will take no action until this sequence is
885 received.
887 The syntax for each command is shown with the discussion of that
888 command. Common elements and parameters are shown in Section 4.1.2.
890 Commands and replies are composed of characters from the ASCII
891 character set [6]. When the transport service provides an 8-bit byte
892 (octet) transmission channel, each 7-bit character is transmitted,
893 right justified, in an octet with the high-order bit cleared to zero.
894 More specifically, the unextended SMTP service provides 7-bit
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903 transport only. An originating SMTP client that has not successfully
904 negotiated an appropriate extension with a particular server (see the
905 next paragraph) MUST NOT transmit messages with information in the
906 high-order bit of octets. If such messages are transmitted in
907 violation of this rule, receiving SMTP servers MAY clear the high-
908 order bit or reject the message as invalid. In general, a relay SMTP
909 SHOULD assume that the message content it has received is valid and,
910 assuming that the envelope permits doing so, relay it without
911 inspecting that content. Of course, if the content is mislabeled and
912 the data path cannot accept the actual content, this may result in
913 the ultimate delivery of a severely garbled message to the recipient.
914 Delivery SMTP systems MAY reject such messages, or return them as
915 undeliverable, rather than deliver them. In the absence of a server-
916 offered extension explicitly permitting it, a sending SMTP system is
917 not permitted to send envelope commands in any character set other
918 than US-ASCII. Receiving systems SHOULD reject such commands,
919 normally using "500 syntax error - invalid character" replies.
921 8-bit message content transmission MAY be requested of the server by
922 a client using extended SMTP facilities, notably the "8BITMIME"
923 extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
924 servers. However, it MUST NOT be construed as authorization to
925 transmit unrestricted 8-bit material, nor does 8BITMIME authorize
926 transmission of any envelope material in other than ASCII. 8BITMIME
927 MUST NOT be requested by senders for material with the high bit on
928 that is not in MIME format with an appropriate content-transfer
929 encoding; servers MAY reject such messages.
931 The metalinguistic notation used in this document corresponds to the
932 "Augmented BNF" used in other Internet mail system documents. The
933 reader who is not familiar with that syntax should consult the ABNF
934 specification in RFC 5234 [7]. Metalanguage terms used in running
935 text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
936 The reader is cautioned that the grammar expressed in the
937 metalanguage is not comprehensive. There are many instances in which
938 provisions in the text constrain or otherwise modify the syntax or
939 semantics implied by the grammar.
941 3. The SMTP Procedures: An Overview
943 This section contains descriptions of the procedures used in SMTP:
944 session initiation, mail transaction, forwarding mail, verifying
945 mailbox names and expanding mailing lists, and opening and closing
946 exchanges. Comments on relaying, a note on mail domains, and a
947 discussion of changing roles are included at the end of this section.
948 Several complete scenarios are presented in Appendix D.
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959 3.1. Session Initiation
961 An SMTP session is initiated when a client opens a connection to a
962 server and the server responds with an opening message.
964 SMTP server implementations MAY include identification of their
965 software and version information in the connection greeting reply
966 after the 220 code, a practice that permits more efficient isolation
967 and repair of any problems. Implementations MAY make provision for
968 SMTP servers to disable the software and version announcement where
969 it causes security concerns. While some systems also identify their
970 contact point for mail problems, this is not a substitute for
971 maintaining the required "postmaster" address (see Section 4).
973 The SMTP protocol allows a server to formally reject a mail session
974 while still allowing the initial connection as follows: a 554
975 response MAY be given in the initial connection opening message
976 instead of the 220. A server taking this approach MUST still wait
977 for the client to send a QUIT (see Section 4.1.1.10) before closing
978 the connection and SHOULD respond to any intervening commands with
979 "503 bad sequence of commands". Since an attempt to make an SMTP
980 connection to such a system is probably in error, a server returning
981 a 554 response on connection opening SHOULD provide enough
982 information in the reply text to facilitate debugging of the sending
983 system.
985 3.2. Client Initiation
987 Once the server has sent the greeting (welcoming) message and the
988 client has received it, the client normally sends the EHLO command to
989 the server, indicating the client's identity. In addition to opening
990 the session, use of EHLO indicates that the client is able to process
991 service extensions and requests that the server provide a list of the
992 extensions it supports. Older SMTP systems that are unable to
993 support service extensions, and contemporary clients that do not
994 require service extensions in the mail session being initiated, MAY
995 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO-
996 style response to a HELO command. For a particular connection
997 attempt, if the server returns a "command not recognized" response to
998 EHLO, the client SHOULD be able to fall back and send HELO.
1000 In the EHLO command, the host sending the command identifies itself;
1001 the command may be interpreted as saying "Hello, I am <domain>" (and,
1002 in the case of EHLO, "and I support service extension requests").
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1015 3.3. Mail Transactions
1017 There are three steps to SMTP mail transactions. The transaction
1018 starts with a MAIL command that gives the sender identification. (In
1019 general, the MAIL command may be sent only when no mail transaction
1020 is in progress; see Section 4.1.4.) A series of one or more RCPT
1021 commands follows, giving the receiver information. Then, a DATA
1022 command initiates transfer of the mail data and is terminated by the
1023 "end of mail" data indicator, which also confirms the transaction.
1025 The first step in the procedure is the MAIL command.
1027 MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
1029 This command tells the SMTP-receiver that a new mail transaction is
1030 starting and to reset all its state tables and buffers, including any
1031 recipients or mail data. The <reverse-path> portion of the first or
1032 only argument contains the source mailbox (between "<" and ">"
1033 brackets), which can be used to report errors (see Section 4.2 for a
1034 discussion of error reporting). If accepted, the SMTP server returns
1035 a "250 OK" reply. If the mailbox specification is not acceptable for
1036 some reason, the server MUST return a reply indicating whether the
1037 failure is permanent (i.e., will occur again if the client tries to
1038 send the same address again) or temporary (i.e., the address might be
1039 accepted if the client tries again later). Despite the apparent
1040 scope of this requirement, there are circumstances in which the
1041 acceptability of the reverse-path may not be determined until one or
1042 more forward-paths (in RCPT commands) can be examined. In those
1043 cases, the server MAY reasonably accept the reverse-path (with a 250
1044 reply) and then report problems after the forward-paths are received
1045 and examined. Normally, failures produce 550 or 553 replies.
1047 Historically, the <reverse-path> was permitted to contain more than
1048 just a mailbox; however, contemporary systems SHOULD NOT use source
1049 routing (see Appendix C).
1051 The optional <mail-parameters> are associated with negotiated SMTP
1052 service extensions (see Section 2.2).
1054 The second step in the procedure is the RCPT command. This step of
1055 the procedure can be repeated any number of times.
1057 RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
1059 The first or only argument to this command includes a forward-path
1060 (normally a mailbox and domain, always surrounded by "<" and ">"
1061 brackets) identifying one recipient. If accepted, the SMTP server
1062 returns a "250 OK" reply and stores the forward-path. If the
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1071 recipient is known not to be a deliverable address, the SMTP server
1072 returns a 550 reply, typically with a string such as "no such user -
1073 " and the mailbox name (other circumstances and reply codes are
1074 possible).
1076 The <forward-path> can contain more than just a mailbox.
1077 Historically, the <forward-path> was permitted to contain a source
1078 routing list of hosts and the destination mailbox; however,
1079 contemporary SMTP clients SHOULD NOT utilize source routes (see
1080 Appendix C). Servers MUST be prepared to encounter a list of source
1081 routes in the forward-path, but they SHOULD ignore the routes or MAY
1082 decline to support the relaying they imply. Similarly, servers MAY
1083 decline to accept mail that is destined for other hosts or systems.
1084 These restrictions make a server useless as a relay for clients that
1085 do not support full SMTP functionality. Consequently, restricted-
1086 capability clients MUST NOT assume that any SMTP server on the
1087 Internet can be used as their mail processing (relaying) site. If a
1088 RCPT command appears without a previous MAIL command, the server MUST
1089 return a 503 "Bad sequence of commands" response. The optional
1090 <rcpt-parameters> are associated with negotiated SMTP service
1091 extensions (see Section 2.2).
1093 Since it has been a common source of errors, it is worth noting that
1094 spaces are not permitted on either side of the colon following FROM
1095 in the MAIL command or TO in the RCPT command. The syntax is exactly
1096 as given above.
1098 The third step in the procedure is the DATA command (or some
1099 alternative specified in a service extension).
1101 DATA <CRLF>
1103 If accepted, the SMTP server returns a 354 Intermediate reply and
1104 considers all succeeding lines up to but not including the end of
1105 mail data indicator to be the message text. When the end of text is
1106 successfully received and stored, the SMTP-receiver sends a "250 OK"
1107 reply.
1109 Since the mail data is sent on the transmission channel, the end of
1110 mail data must be indicated so that the command and reply dialog can
1111 be resumed. SMTP indicates the end of the mail data by sending a
1112 line containing only a "." (period or full stop). A transparency
1113 procedure is used to prevent this from interfering with the user's
1114 text (see Section 4.5.2).
1116 The end of mail data indicator also confirms the mail transaction and
1117 tells the SMTP server to now process the stored recipients and mail
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1127 data. If accepted, the SMTP server returns a "250 OK" reply. The
1128 DATA command can fail at only two points in the protocol exchange:
1130 If there was no MAIL, or no RCPT, command, or all such commands were
1131 rejected, the server MAY return a "command out of sequence" (503) or
1132 "no valid recipients" (554) reply in response to the DATA command.
1133 If one of those replies (or any other 5yz reply) is received, the
1134 client MUST NOT send the message data; more generally, message data
1135 MUST NOT be sent unless a 354 reply is received.
1137 If the verb is initially accepted and the 354 reply issued, the DATA
1138 command should fail only if the mail transaction was incomplete (for
1139 example, no recipients), if resources were unavailable (including, of
1140 course, the server unexpectedly becoming unavailable), or if the
1141 server determines that the message should be rejected for policy or
1142 other reasons.
1144 However, in practice, some servers do not perform recipient
1145 verification until after the message text is received. These servers
1146 SHOULD treat a failure for one or more recipients as a "subsequent
1147 failure" and return a mail message as discussed in Section 6 and, in
1148 particular, in Section 6.1. Using a "550 mailbox not found" (or
1149 equivalent) reply code after the data are accepted makes it difficult
1150 or impossible for the client to determine which recipients failed.
1152 When the RFC 822 format ([28], [4]) is being used, the mail data
1153 include the header fields such as those named Date, Subject, To, Cc,
1154 and From. Server SMTP systems SHOULD NOT reject messages based on
1155 perceived defects in the RFC 822 or MIME (RFC 2045 [21]) message
1156 header section or message body. In particular, they MUST NOT reject
1157 messages in which the numbers of Resent-header fields do not match or
1158 Resent-to appears without Resent-from and/or Resent-date.
1160 Mail transaction commands MUST be used in the order discussed above.
1162 3.4. Forwarding for Address Correction or Updating
1164 Forwarding support is most often required to consolidate and simplify
1165 addresses within, or relative to, some enterprise and less frequently
1166 to establish addresses to link a person's prior address with a
1167 current one. Silent forwarding of messages (without server
1168 notification to the sender), for security or non-disclosure purposes,
1169 is common in the contemporary Internet.
1171 In both the enterprise and the "new address" cases, information
1172 hiding (and sometimes security) considerations argue against exposure
1173 of the "final" address through the SMTP protocol as a side effect of
1174 the forwarding activity. This may be especially important when the
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1183 final address may not even be reachable by the sender. Consequently,
1184 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and
1185 especially the 251 (corrected destination) and 551 reply codes from
1186 RCPT must be evaluated carefully by implementers and, when they are
1187 available, by those configuring systems (see also Section 7.4).
1189 In particular:
1191 o Servers MAY forward messages when they are aware of an address
1192 change. When they do so, they MAY either provide address-updating
1193 information with a 251 code, or may forward "silently" and return
1194 a 250 code. However, if a 251 code is used, they MUST NOT assume
1195 that the client will actually update address information or even
1196 return that information to the user.
1198 Alternately,
1200 o Servers MAY reject messages or return them as non-deliverable when
1201 they cannot be delivered precisely as addressed. When they do so,
1202 they MAY either provide address-updating information with a 551
1203 code, or may reject the message as undeliverable with a 550 code
1204 and no address-specific information. However, if a 551 code is
1205 used, they MUST NOT assume that the client will actually update
1206 address information or even return that information to the user.
1208 SMTP server implementations that support the 251 and/or 551 reply
1209 codes SHOULD provide configuration mechanisms so that sites that
1210 conclude that they would undesirably disclose information can disable
1211 or restrict their use.
1213 3.5. Commands for Debugging Addresses
1215 3.5.1. Overview
1217 SMTP provides commands to verify a user name or obtain the content of
1218 a mailing list. This is done with the VRFY and EXPN commands, which
1219 have character string arguments. Implementations SHOULD support VRFY
1220 and EXPN (however, see Section 3.5.2 and Section 7.3).
1222 For the VRFY command, the string is a user name or a user name and
1223 domain (see below). If a normal (i.e., 250) response is returned,
1224 the response MAY include the full name of the user and MUST include
1225 the mailbox of the user. It MUST be in either of the following
1226 forms:
1228 User Name <local-part@domain>
1229 local-part@domain
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1239 When a name that is the argument to VRFY could identify more than one
1240 mailbox, the server MAY either note the ambiguity or identify the
1241 alternatives. In other words, any of the following are legitimate
1242 responses to VRFY:
1244 553 User ambiguous
1246 or
1248 553- Ambiguous; Possibilities are
1249 553-Joe Smith <jsmith@foo.com>
1250 553-Harry Smith <hsmith@foo.com>
1251 553 Melvin Smith <dweep@foo.com>
1253 or
1255 553-Ambiguous; Possibilities
1256 553- <jsmith@foo.com>
1257 553- <hsmith@foo.com>
1258 553 <dweep@foo.com>
1260 Under normal circumstances, a client receiving a 553 reply would be
1261 expected to expose the result to the user. Use of exactly the forms
1262 given, and the "user ambiguous" or "ambiguous" keywords, possibly
1263 supplemented by extended reply codes, such as those described in RFC
1264 3463 [25], will facilitate automated translation into other languages
1265 as needed. Of course, a client that was highly automated or that was
1266 operating in another language than English might choose to try to
1267 translate the response to return some other indication to the user
1268 than the literal text of the reply, or to take some automated action
1269 such as consulting a directory service for additional information
1270 before reporting to the user.
1272 For the EXPN command, the string identifies a mailing list, and the
1273 successful (i.e., 250) multiline response MAY include the full name
1274 of the users and MUST give the mailboxes on the mailing list.
1276 In some hosts, the distinction between a mailing list and an alias
1277 for a single mailbox is a bit fuzzy, since a common data structure
1278 may hold both types of entries, and it is possible to have mailing
1279 lists containing only one mailbox. If a request is made to apply
1280 VRFY to a mailing list, a positive response MAY be given if a message
1281 so addressed would be delivered to everyone on the list, otherwise an
1282 error SHOULD be reported (e.g., "550 That is a mailing list, not a
1283 user" or "252 Unable to verify members of mailing list"). If a
1284 request is made to expand a user name, the server MAY return a
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1292 RFC 5321 SMTP October 2008
1295 positive response consisting of a list containing one name, or an
1296 error MAY be reported (e.g., "550 That is a user name, not a mailing
1297 list").
1299 In the case of a successful multiline reply (normal for EXPN),
1300 exactly one mailbox is to be specified on each line of the reply.
1301 The case of an ambiguous request is discussed above.
1303 "User name" is a fuzzy term and has been used deliberately. An
1304 implementation of the VRFY or EXPN commands MUST include at least
1305 recognition of local mailboxes as "user names". However, since
1306 current Internet practice often results in a single host handling
1307 mail for multiple domains, hosts, especially hosts that provide this
1308 functionality, SHOULD accept the "local-part@domain" form as a "user
1309 name"; hosts MAY also choose to recognize other strings as "user
1310 names".
1312 The case of expanding a mailbox list requires a multiline reply, such
1313 as:
1315 C: EXPN Example-People
1316 S: 250-Jon Postel <Postel@isi.edu>
1317 S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
1318 S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
1320 or
1322 C: EXPN Executive-Washroom-List
1323 S: 550 Access Denied to You.
1325 The character string arguments of the VRFY and EXPN commands cannot
1326 be further restricted due to the variety of implementations of the
1327 user name and mailbox list concepts. On some systems, it may be
1328 appropriate for the argument of the EXPN command to be a file name
1329 for a file containing a mailing list, but again there are a variety
1330 of file naming conventions in the Internet. Similarly, historical
1331 variations in what is returned by these commands are such that the
1332 response SHOULD be interpreted very carefully, if at all, and SHOULD
1333 generally only be used for diagnostic purposes.
1335 3.5.2. VRFY Normal Response
1337 When normal (2yz or 551) responses are returned from a VRFY or EXPN
1338 request, the reply MUST include the <Mailbox> name using a
1339 "<local-part@domain>" construction, where "domain" is a fully-
1340 qualified domain name. In circumstances exceptional enough to
1341 justify violating the intent of this specification, free-form text
1342 MAY be returned. In order to facilitate parsing by both computers
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1348 RFC 5321 SMTP October 2008
1351 and people, addresses SHOULD appear in pointed brackets. When
1352 addresses, rather than free-form debugging information, are returned,
1353 EXPN and VRFY MUST return only valid domain addresses that are usable
1354 in SMTP RCPT commands. Consequently, if an address implies delivery
1355 to a program or other system, the mailbox name used to reach that
1356 target MUST be given. Paths (explicit source routes) MUST NOT be
1357 returned by VRFY or EXPN.
1359 Server implementations SHOULD support both VRFY and EXPN. For
1360 security reasons, implementations MAY provide local installations a
1361 way to disable either or both of these commands through configuration
1362 options or the equivalent (see Section 7.3). When these commands are
1363 supported, they are not required to work across relays when relaying
1364 is supported. Since they were both optional in RFC 821, but VRFY was
1365 made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be
1366 listed as a service extension in an EHLO response. VRFY MAY be
1367 listed as a convenience but, since support for it is required, SMTP
1368 clients are not required to check for its presence on the extension
1369 list before using it.
1371 3.5.3. Meaning of VRFY or EXPN Success Response
1373 A server MUST NOT return a 250 code in response to a VRFY or EXPN
1374 command unless it has actually verified the address. In particular,
1375 a server MUST NOT return 250 if all it has done is to verify that the
1376 syntax given is valid. In that case, 502 (Command not implemented)
1377 or 500 (Syntax error, command unrecognized) SHOULD be returned. As
1378 stated elsewhere, implementation (in the sense of actually validating
1379 addresses and returning information) of VRFY and EXPN are strongly
1380 recommended. Hence, implementations that return 500 or 502 for VRFY
1381 are not in full compliance with this specification.
1383 There may be circumstances where an address appears to be valid but
1384 cannot reasonably be verified in real time, particularly when a
1385 server is acting as a mail exchanger for another server or domain.
1386 "Apparent validity", in this case, would normally involve at least
1387 syntax checking and might involve verification that any domains
1388 specified were ones to which the host expected to be able to relay
1389 mail. In these situations, reply code 252 SHOULD be returned. These
1390 cases parallel the discussion of RCPT verification in Section 2.1.
1391 Similarly, the discussion in Section 3.4 applies to the use of reply
1392 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are
1393 recognized but that would be forwarded or rejected were mail received
1394 for them. Implementations generally SHOULD be more aggressive about
1395 address verification in the case of VRFY than in the case of RCPT,
1396 even if it takes a little longer to do so.
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1407 3.5.4. Semantics and Applications of EXPN
1409 EXPN is often very useful in debugging and understanding problems
1410 with mailing lists and multiple-target-address aliases. Some systems
1411 have attempted to use source expansion of mailing lists as a means of
1412 eliminating duplicates. The propagation of aliasing systems with
1413 mail on the Internet for hosts (typically with MX and CNAME DNS
1414 records), for mailboxes (various types of local host aliases), and in
1415 various proxying arrangements has made it nearly impossible for these
1416 strategies to work consistently, and mail systems SHOULD NOT attempt
1417 them.
1419 3.6. Relaying and Mail Routing
1421 3.6.1. Source Routes and Relaying
1423 In general, the availability of Mail eXchanger records in the domain
1424 name system (RFC 1035 [2], RFC 974 [12]) makes the use of explicit
1425 source routes in the Internet mail system unnecessary. Many
1426 historical problems with the interpretation of explicit source routes
1427 have made their use undesirable. SMTP clients SHOULD NOT generate
1428 explicit source routes except under unusual circumstances. SMTP
1429 servers MAY decline to act as mail relays or to accept addresses that
1430 specify source routes. When route information is encountered, SMTP
1431 servers MAY ignore the route information and simply send to the final
1432 destination specified as the last element in the route and SHOULD do
1433 so. There has been an invalid practice of using names that do not
1434 appear in the DNS as destination names, with the senders counting on
1435 the intermediate hosts specified in source routing to resolve any
1436 problems. If source routes are stripped, this practice will cause
1437 failures. This is one of several reasons why SMTP clients MUST NOT
1438 generate invalid source routes or depend on serial resolution of
1439 names.
1441 When source routes are not used, the process described in RFC 821 for
1442 constructing a reverse-path from the forward-path is not applicable
1443 and the reverse-path at the time of delivery will simply be the
1444 address that appeared in the MAIL command.
1446 3.6.2. Mail eXchange Records and Relaying
1448 A relay SMTP server is usually the target of a DNS MX record that
1449 designates it, rather than the final delivery system. The relay
1450 server may accept or reject the task of relaying the mail in the same
1451 way it accepts or rejects mail for a local user. If it accepts the
1452 task, it then becomes an SMTP client, establishes a transmission
1453 channel to the next SMTP server specified in the DNS (according to
1454 the rules in Section 5), and sends it the mail. If it declines to
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1460 RFC 5321 SMTP October 2008
1463 relay mail to a particular address for policy reasons, a 550 response
1464 SHOULD be returned.
1466 This specification does not deal with the verification of return
1467 paths for use in delivery notifications. Recent work, such as that
1468 on SPF [29] and DKIM [30] [31], has been done to provide ways to
1469 ascertain that an address is valid or belongs to the person who
1470 actually sent the message. A server MAY attempt to verify the return
1471 path before using its address for delivery notifications, but methods
1472 of doing so are not defined here nor is any particular method
1473 recommended at this time.
1475 3.6.3. Message Submission Servers as Relays
1477 Many mail-sending clients exist, especially in conjunction with
1478 facilities that receive mail via POP3 or IMAP, that have limited
1479 capability to support some of the requirements of this specification,
1480 such as the ability to queue messages for subsequent delivery
1481 attempts. For these clients, it is common practice to make private
1482 arrangements to send all messages to a single server for processing
1483 and subsequent distribution. SMTP, as specified here, is not ideally
1484 suited for this role. A standardized mail submission protocol has
1485 been developed that is gradually superseding practices based on SMTP
1486 (see RFC 4409 [18]). In any event, because these arrangements are
1487 private and fall outside the scope of this specification, they are
1488 not described here.
1490 It is important to note that MX records can point to SMTP servers
1491 that act as gateways into other environments, not just SMTP relays
1492 and final delivery systems; see Sections 3.7 and 5.
1494 If an SMTP server has accepted the task of relaying the mail and
1495 later finds that the destination is incorrect or that the mail cannot
1496 be delivered for some other reason, then it MUST construct an
1497 "undeliverable mail" notification message and send it to the
1498 originator of the undeliverable mail (as indicated by the reverse-
1499 path). Formats specified for non-delivery reports by other standards
1500 (see, for example, RFC 3461 [32] and RFC 3464 [33]) SHOULD be used if
1501 possible.
1503 This notification message must be from the SMTP server at the relay
1504 host or the host that first determines that delivery cannot be
1505 accomplished. Of course, SMTP servers MUST NOT send notification
1506 messages about problems transporting notification messages. One way
1507 to prevent loops in error reporting is to specify a null reverse-path
1508 in the MAIL command of a notification message. When such a message
1509 is transmitted, the reverse-path MUST be set to null (see
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1516 RFC 5321 SMTP October 2008
1519 Section 4.5.5 for additional discussion). A MAIL command with a null
1520 reverse-path appears as follows:
1522 MAIL FROM:<>
1524 As discussed in Section 6.4, a relay SMTP has no need to inspect or
1525 act upon the header section or body of the message data and MUST NOT
1526 do so except to add its own "Received:" header field (Section 4.4)
1527 and, optionally, to attempt to detect looping in the mail system (see
1528 Section 6.3). Of course, this prohibition also applies to any
1529 modifications of these header fields or text (see also Section 7.9).
1531 3.7. Mail Gatewaying
1533 While the relay function discussed above operates within the Internet
1534 SMTP transport service environment, MX records or various forms of
1535 explicit routing may require that an intermediate SMTP server perform
1536 a translation function between one transport service and another. As
1537 discussed in Section 2.3.10, when such a system is at the boundary
1538 between two transport service environments, we refer to it as a
1539 "gateway" or "gateway SMTP".
1541 Gatewaying mail between different mail environments, such as
1542 different mail formats and protocols, is complex and does not easily
1543 yield to standardization. However, some general requirements may be
1544 given for a gateway between the Internet and another mail
1545 environment.
1547 3.7.1. Header Fields in Gatewaying
1549 Header fields MAY be rewritten when necessary as messages are
1550 gatewayed across mail environment boundaries. This may involve
1551 inspecting the message body or interpreting the local-part of the
1552 destination address in spite of the prohibitions in Section 6.4.
1554 Other mail systems gatewayed to the Internet often use a subset of
1555 the RFC 822 header section or provide similar functionality with a
1556 different syntax, but some of these mail systems do not have an
1557 equivalent to the SMTP envelope. Therefore, when a message leaves
1558 the Internet environment, it may be necessary to fold the SMTP
1559 envelope information into the message header section. A possible
1560 solution would be to create new header fields to carry the envelope
1561 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this
1562 would require changes in mail programs in foreign environments and
1563 might risk disclosure of private information (see Section 7.2).
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1575 3.7.2. Received Lines in Gatewaying
1577 When forwarding a message into or out of the Internet environment, a
1578 gateway MUST prepend a Received: line, but it MUST NOT alter in any
1579 way a Received: line that is already in the header section.
1581 "Received:" header fields of messages originating from other
1582 environments may not conform exactly to this specification. However,
1583 the most important use of Received: lines is for debugging mail
1584 faults, and this debugging can be severely hampered by well-meaning
1585 gateways that try to "fix" a Received: line. As another consequence
1586 of trace header fields arising in non-SMTP environments, receiving
1587 systems MUST NOT reject mail based on the format of a trace header
1588 field and SHOULD be extremely robust in the light of unexpected
1589 information or formats in those header fields.
1591 The gateway SHOULD indicate the environment and protocol in the "via"
1592 clauses of Received header field(s) that it supplies.
1594 3.7.3. Addresses in Gatewaying
1596 From the Internet side, the gateway SHOULD accept all valid address
1597 formats in SMTP commands and in the RFC 822 header section, and all
1598 valid RFC 822 messages. Addresses and header fields generated by
1599 gateways MUST conform to applicable standards (including this one and
1600 RFC 5322 [4]). Gateways are, of course, subject to the same rules
1601 for handling source routes as those described for other SMTP systems
1602 in Section 3.3.
1604 3.7.4. Other Header Fields in Gatewaying
1606 The gateway MUST ensure that all header fields of a message that it
1607 forwards into the Internet mail environment meet the requirements for
1608 Internet mail. In particular, all addresses in "From:", "To:",
1609 "Cc:", etc., header fields MUST be transformed (if necessary) to
1610 satisfy the standard header syntax of RFC 5322 [4], MUST reference
1611 only fully-qualified domain names, and MUST be effective and useful
1612 for sending replies. The translation algorithm used to convert mail
1613 from the Internet protocols to another environment's protocol SHOULD
1614 ensure that error messages from the foreign mail environment are
1615 delivered to the reverse-path from the SMTP envelope, not to an
1616 address in the "From:", "Sender:", or similar header fields of the
1617 message.
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1628 RFC 5321 SMTP October 2008
1631 3.7.5. Envelopes in Gatewaying
1633 Similarly, when forwarding a message from another environment into
1634 the Internet, the gateway SHOULD set the envelope return path in
1635 accordance with an error message return address, if supplied by the
1636 foreign environment. If the foreign environment has no equivalent
1637 concept, the gateway must select and use a best approximation, with
1638 the message originator's address as the default of last resort.
1640 3.8. Terminating Sessions and Connections
1642 An SMTP connection is terminated when the client sends a QUIT
1643 command. The server responds with a positive reply code, after which
1644 it closes the connection.
1646 An SMTP server MUST NOT intentionally close the connection under
1647 normal operational circumstances (see Section 7.8) except:
1649 o After receiving a QUIT command and responding with a 221 reply.
1651 o After detecting the need to shut down the SMTP service and
1652 returning a 421 response code. This response code can be issued
1653 after the server receives any command or, if necessary,
1654 asynchronously from command receipt (on the assumption that the
1655 client will receive it after the next command is issued).
1657 o After a timeout, as specified in Section 4.5.3.2, occurs waiting
1658 for the client to send a command or data.
1660 In particular, a server that closes connections in response to
1661 commands that are not understood is in violation of this
1662 specification. Servers are expected to be tolerant of unknown
1663 commands, issuing a 500 reply and awaiting further instructions from
1664 the client.
1666 An SMTP server that is forcibly shut down via external means SHOULD
1667 attempt to send a line containing a 421 response code to the SMTP
1668 client before exiting. The SMTP client will normally read the 421
1669 response code after sending its next command.
1671 SMTP clients that experience a connection close, reset, or other
1672 communications failure due to circumstances not under their control
1673 (in violation of the intent of this specification but sometimes
1674 unavoidable) SHOULD, to maintain the robustness of the mail system,
1675 treat the mail transaction as if a 451 response had been received and
1676 act accordingly.
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1684 RFC 5321 SMTP October 2008
1687 3.9. Mailing Lists and Aliases
1689 An SMTP-capable host SHOULD support both the alias and the list
1690 models of address expansion for multiple delivery. When a message is
1691 delivered or forwarded to each address of an expanded list form, the
1692 return address in the envelope ("MAIL FROM:") MUST be changed to be
1693 the address of a person or other entity who administers the list.
1694 However, in this case, the message header section (RFC 5322 [4]) MUST
1695 be left unchanged; in particular, the "From" field of the header
1696 section is unaffected.
1698 An important mail facility is a mechanism for multi-destination
1699 delivery of a single message, by transforming (or "expanding" or
1700 "exploding") a pseudo-mailbox address into a list of destination
1701 mailbox addresses. When a message is sent to such a pseudo-mailbox
1702 (sometimes called an "exploder"), copies are forwarded or
1703 redistributed to each mailbox in the expanded list. Servers SHOULD
1704 simply utilize the addresses on the list; application of heuristics
1705 or other matching rules to eliminate some addresses, such as that of
1706 the originator, is strongly discouraged. We classify such a pseudo-
1707 mailbox as an "alias" or a "list", depending upon the expansion
1708 rules.
1710 3.9.1. Alias
1712 To expand an alias, the recipient mailer simply replaces the pseudo-
1713 mailbox address in the envelope with each of the expanded addresses
1714 in turn; the rest of the envelope and the message body are left
1715 unchanged. The message is then delivered or forwarded to each
1716 expanded address.
1718 3.9.2. List
1720 A mailing list may be said to operate by "redistribution" rather than
1721 by "forwarding". To expand a list, the recipient mailer replaces the
1722 pseudo-mailbox address in the envelope with each of the expanded
1723 addresses in turn. The return (backward-pointing) address in the
1724 envelope is changed so that all error messages generated by the final
1725 deliveries will be returned to a list administrator, not to the
1726 message originator, who generally has no control over the contents of
1727 the list and will typically find error messages annoying. Note that
1728 the key difference between handling aliases (Section 3.9.1) and
1729 forwarding (this subsection) is the change to the backward-pointing
1730 address in this case. When a list constrains its processing to the
1731 very limited set of modifications and actions described here, it is
1732 attempting to emulate an MTA; such lists can be treated as a
1733 continuation in email transit.
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1743 There exist mailing lists that perform additional, sometimes
1744 extensive, modifications to a message and its envelope. Such mailing
1745 lists need to be viewed as full MUAs, which accept a delivery and
1746 post a new message.
1748 4. The SMTP Specifications
1750 4.1. SMTP Commands
1752 4.1.1. Command Semantics and Syntax
1754 The SMTP commands define the mail transfer or the mail system
1755 function requested by the user. SMTP commands are character strings
1756 terminated by <CRLF>. The commands themselves are alphabetic
1757 characters terminated by <SP> if parameters follow and <CRLF>
1758 otherwise. (In the interest of improved interoperability, SMTP
1759 receivers SHOULD tolerate trailing white space before the terminating
1760 <CRLF>.) The syntax of the local part of a mailbox MUST conform to
1761 receiver site conventions and the syntax specified in Section 4.1.2.
1762 The SMTP commands are discussed below. The SMTP replies are
1763 discussed in Section 4.2.
1765 A mail transaction involves several data objects that are
1766 communicated as arguments to different commands. The reverse-path is
1767 the argument of the MAIL command, the forward-path is the argument of
1768 the RCPT command, and the mail data is the argument of the DATA
1769 command. These arguments or data objects must be transmitted and
1770 held, pending the confirmation communicated by the end of mail data
1771 indication that finalizes the transaction. The model for this is
1772 that distinct buffers are provided to hold the types of data objects;
1773 that is, there is a reverse-path buffer, a forward-path buffer, and a
1774 mail data buffer. Specific commands cause information to be appended
1775 to a specific buffer, or cause one or more buffers to be cleared.
1777 Several commands (RSET, DATA, QUIT) are specified as not permitting
1778 parameters. In the absence of specific extensions offered by the
1779 server and accepted by the client, clients MUST NOT send such
1780 parameters and servers SHOULD reject commands containing them as
1781 having invalid syntax.
1783 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)
1785 These commands are used to identify the SMTP client to the SMTP
1786 server. The argument clause contains the fully-qualified domain name
1787 of the SMTP client, if one is available. In situations in which the
1788 SMTP client system does not have a meaningful domain name (e.g., when
1789 its address is dynamically allocated and no reverse mapping record is
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1796 RFC 5321 SMTP October 2008
1799 available), the client SHOULD send an address literal (see
1800 Section 4.1.3).
1802 RFC 2821, and some earlier informal practices, encouraged following
1803 the literal by information that would help to identify the client
1804 system. That convention was not widely supported, and many SMTP
1805 servers considered it an error. In the interest of interoperability,
1806 it is probably wise for servers to be prepared for this string to
1807 occur, but SMTP clients SHOULD NOT send it.
1809 The SMTP server identifies itself to the SMTP client in the
1810 connection greeting reply and in the response to this command.
1812 A client SMTP SHOULD start an SMTP session by issuing the EHLO
1813 command. If the SMTP server supports the SMTP service extensions, it
1814 will give a successful response, a failure response, or an error
1815 response. If the SMTP server, in violation of this specification,
1816 does not support any SMTP service extensions, it will generate an
1817 error response. Older client SMTP systems MAY, as discussed above,
1818 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
1819 support the HELO command and reply properly to it. In any event, a
1820 client MUST issue HELO or EHLO before starting a mail transaction.
1822 These commands, and a "250 OK" reply to one of them, confirm that
1823 both the SMTP client and the SMTP server are in the initial state,
1824 that is, there is no transaction in progress and all state tables and
1825 buffers are cleared.
1827 Syntax:
1829 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF
1831 helo = "HELO" SP Domain CRLF
1833 Normally, the response to EHLO will be a multiline reply. Each line
1834 of the response contains a keyword and, optionally, one or more
1835 parameters. Following the normal syntax for multiline replies, these
1836 keywords follow the code (250) and a hyphen for all but the last
1837 line, and the code and a space for the last line. The syntax for a
1838 positive response, using the ABNF notation and terminal symbols of
1839 RFC 5234 [7], is:
1841 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
1842 / ( "250-" Domain [ SP ehlo-greet ] CRLF
1843 *( "250-" ehlo-line CRLF )
1844 "250" SP ehlo-line CRLF )
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1855 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)
1856 ; string of any characters other than CR or LF
1858 ehlo-line = ehlo-keyword *( SP ehlo-param )
1860 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
1861 ; additional syntax of ehlo-params depends on
1862 ; ehlo-keyword
1864 ehlo-param = 1*(%d33-126)
1865 ; any CHAR excluding <SP> and all
1866 ; control characters (US-ASCII 0-31 and 127
1867 ; inclusive)
1869 Although EHLO keywords may be specified in upper, lower, or mixed
1870 case, they MUST always be recognized and processed in a case-
1871 insensitive manner. This is simply an extension of practices
1872 specified in RFC 821 and Section 2.4.
1874 The EHLO response MUST contain keywords (and associated parameters if
1875 required) for all commands not listed as "required" in Section 4.5.1
1876 excepting only private-use commands as described in Section 4.1.5.
1877 Private-use commands MAY be listed.
1879 4.1.1.2. MAIL (MAIL)
1881 This command is used to initiate a mail transaction in which the mail
1882 data is delivered to an SMTP server that may, in turn, deliver it to
1883 one or more mailboxes or pass it on to another system (possibly using
1884 SMTP). The argument clause contains a reverse-path and may contain
1885 optional parameters. In general, the MAIL command may be sent only
1886 when no mail transaction is in progress, see Section 4.1.4.
1888 The reverse-path consists of the sender mailbox. Historically, that
1889 mailbox might optionally have been preceded by a list of hosts, but
1890 that behavior is now deprecated (see Appendix C). In some types of
1891 reporting messages for which a reply is likely to cause a mail loop
1892 (for example, mail delivery and non-delivery notifications), the
1893 reverse-path may be null (see Section 3.6).
1895 This command clears the reverse-path buffer, the forward-path buffer,
1896 and the mail data buffer, and it inserts the reverse-path information
1897 from its argument clause into the reverse-path buffer.
1899 If service extensions were negotiated, the MAIL command may also
1900 carry parameters associated with a particular service extension.
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1908 RFC 5321 SMTP October 2008
1911 Syntax:
1913 mail = "MAIL FROM:" Reverse-path
1914 [SP Mail-parameters] CRLF
1916 4.1.1.3. RECIPIENT (RCPT)
1918 This command is used to identify an individual recipient of the mail
1919 data; multiple recipients are specified by multiple uses of this
1920 command. The argument clause contains a forward-path and may contain
1921 optional parameters.
1923 The forward-path normally consists of the required destination
1924 mailbox. Sending systems SHOULD NOT generate the optional list of
1925 hosts known as a source route. Receiving systems MUST recognize
1926 source route syntax but SHOULD strip off the source route
1927 specification and utilize the domain name associated with the mailbox
1928 as if the source route had not been provided.
1930 Similarly, relay hosts SHOULD strip or ignore source routes, and
1931 names MUST NOT be copied into the reverse-path. When mail reaches
1932 its ultimate destination (the forward-path contains only a
1933 destination mailbox), the SMTP server inserts it into the destination
1934 mailbox in accordance with its host mail conventions.
1936 This command appends its forward-path argument to the forward-path
1937 buffer; it does not change the reverse-path buffer nor the mail data
1938 buffer.
1940 For example, mail received at relay host xyz.com with envelope
1941 commands
1943 MAIL FROM:<userx@y.foo.org>
1944 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
1946 will normally be sent directly on to host d.bar.org with envelope
1947 commands
1949 MAIL FROM:<userx@y.foo.org>
1950 RCPT TO:<userc@d.bar.org>
1952 As provided in Appendix C, xyz.com MAY also choose to relay the
1953 message to hosta.int, using the envelope commands
1955 MAIL FROM:<userx@y.foo.org>
1956 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
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1964 RFC 5321 SMTP October 2008
1967 or to jkl.org, using the envelope commands
1969 MAIL FROM:<userx@y.foo.org>
1970 RCPT TO:<@jkl.org:userc@d.bar.org>
1972 Attempting to use relaying this way is now strongly discouraged.
1973 Since hosts are not required to relay mail at all, xyz.com MAY also
1974 reject the message entirely when the RCPT command is received, using
1975 a 550 code (since this is a "policy reason").
1977 If service extensions were negotiated, the RCPT command may also
1978 carry parameters associated with a particular service extension
1979 offered by the server. The client MUST NOT transmit parameters other
1980 than those associated with a service extension offered by the server
1981 in its EHLO response.
1983 Syntax:
1985 rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" /
1986 Forward-path ) [SP Rcpt-parameters] CRLF
1988 Note that, in a departure from the usual rules for
1989 local-parts, the "Postmaster" string shown above is
1990 treated as case-insensitive.
1992 4.1.1.4. DATA (DATA)
1994 The receiver normally sends a 354 response to DATA, and then treats
1995 the lines (strings ending in <CRLF> sequences, as described in
1996 Section 2.3.7) following the command as mail data from the sender.
1997 This command causes the mail data to be appended to the mail data
1998 buffer. The mail data may contain any of the 128 ASCII character
1999 codes, although experience has indicated that use of control
2000 characters other than SP, HT, CR, and LF may cause problems and
2001 SHOULD be avoided when possible.
2003 The mail data are terminated by a line containing only a period, that
2004 is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
2005 actually the terminator of the previous line (see Section 4.5.2).
2006 This is the end of mail data indication. The first <CRLF> of this
2007 terminating sequence is also the <CRLF> that ends the final line of
2008 the data (message text) or, if there was no mail data, ends the DATA
2009 command itself (the "no mail data" case does not conform to this
2010 specification since it would require that neither the trace header
2011 fields required by this specification nor the message header section
2012 required by RFC 5322 [4] be transmitted). An extra <CRLF> MUST NOT
2013 be added, as that would cause an empty line to be added to the
2014 message. The only exception to this rule would arise if the message
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2020 RFC 5321 SMTP October 2008
2023 body were passed to the originating SMTP-sender with a final "line"
2024 that did not end in <CRLF>; in that case, the originating SMTP system
2025 MUST either reject the message as invalid or add <CRLF> in order to
2026 have the receiving SMTP server recognize the "end of data" condition.
2028 The custom of accepting lines ending only in <LF>, as a concession to
2029 non-conforming behavior on the part of some UNIX systems, has proven
2030 to cause more interoperability problems than it solves, and SMTP
2031 server systems MUST NOT do this, even in the name of improved
2032 robustness. In particular, the sequence "<LF>.<LF>" (bare line
2033 feeds, without carriage returns) MUST NOT be treated as equivalent to
2034 <CRLF>.<CRLF> as the end of mail data indication.
2036 Receipt of the end of mail data indication requires the server to
2037 process the stored mail transaction information. This processing
2038 consumes the information in the reverse-path buffer, the forward-path
2039 buffer, and the mail data buffer, and on the completion of this
2040 command these buffers are cleared. If the processing is successful,
2041 the receiver MUST send an OK reply. If the processing fails, the
2042 receiver MUST send a failure reply. The SMTP model does not allow
2043 for partial failures at this point: either the message is accepted by
2044 the server for delivery and a positive response is returned or it is
2045 not accepted and a failure reply is returned. In sending a positive
2046 "250 OK" completion reply to the end of data indication, the receiver
2047 takes full responsibility for the message (see Section 6.1). Errors
2048 that are diagnosed subsequently MUST be reported in a mail message,
2049 as discussed in Section 4.4.
2051 When the SMTP server accepts a message either for relaying or for
2052 final delivery, it inserts a trace record (also referred to
2053 interchangeably as a "time stamp line" or "Received" line) at the top
2054 of the mail data. This trace record indicates the identity of the
2055 host that sent the message, the identity of the host that received
2056 the message (and is inserting this time stamp), and the date and time
2057 the message was received. Relayed messages will have multiple time
2058 stamp lines. Details for formation of these lines, including their
2059 syntax, is specified in Section 4.4.
2061 Additional discussion about the operation of the DATA command appears
2062 in Section 3.3.
2064 Syntax:
2066 data = "DATA" CRLF
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2076 RFC 5321 SMTP October 2008
2079 4.1.1.5. RESET (RSET)
2081 This command specifies that the current mail transaction will be
2082 aborted. Any stored sender, recipients, and mail data MUST be
2083 discarded, and all buffers and state tables cleared. The receiver
2084 MUST send a "250 OK" reply to a RSET command with no arguments. A
2085 reset command may be issued by the client at any time. It is
2086 effectively equivalent to a NOOP (i.e., it has no effect) if issued
2087 immediately after EHLO, before EHLO is issued in the session, after
2088 an end of data indicator has been sent and acknowledged, or
2089 immediately before a QUIT. An SMTP server MUST NOT close the
2090 connection as the result of receiving a RSET; that action is reserved
2091 for QUIT (see Section 4.1.1.10).
2093 Since EHLO implies some additional processing and response by the
2094 server, RSET will normally be more efficient than reissuing that
2095 command, even though the formal semantics are the same.
2097 There are circumstances, contrary to the intent of this
2098 specification, in which an SMTP server may receive an indication that
2099 the underlying TCP connection has been closed or reset. To preserve
2100 the robustness of the mail system, SMTP servers SHOULD be prepared
2101 for this condition and SHOULD treat it as if a QUIT had been received
2102 before the connection disappeared.
2104 Syntax:
2106 rset = "RSET" CRLF
2108 4.1.1.6. VERIFY (VRFY)
2110 This command asks the receiver to confirm that the argument
2111 identifies a user or mailbox. If it is a user name, information is
2112 returned as specified in Section 3.5.
2114 This command has no effect on the reverse-path buffer, the forward-
2115 path buffer, or the mail data buffer.
2117 Syntax:
2119 vrfy = "VRFY" SP String CRLF
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2132 RFC 5321 SMTP October 2008
2135 4.1.1.7. EXPAND (EXPN)
2137 This command asks the receiver to confirm that the argument
2138 identifies a mailing list, and if so, to return the membership of
2139 that list. If the command is successful, a reply is returned
2140 containing information as described in Section 3.5. This reply will
2141 have multiple lines except in the trivial case of a one-member list.
2143 This command has no effect on the reverse-path buffer, the forward-
2144 path buffer, or the mail data buffer, and it may be issued at any
2145 time.
2147 Syntax:
2149 expn = "EXPN" SP String CRLF
2151 4.1.1.8. HELP (HELP)
2153 This command causes the server to send helpful information to the
2154 client. The command MAY take an argument (e.g., any command name)
2155 and return more specific information as a response.
2157 This command has no effect on the reverse-path buffer, the forward-
2158 path buffer, or the mail data buffer, and it may be issued at any
2159 time.
2161 SMTP servers SHOULD support HELP without arguments and MAY support it
2162 with arguments.
2164 Syntax:
2166 help = "HELP" [ SP String ] CRLF
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2191 4.1.1.9. NOOP (NOOP)
2193 This command does not affect any parameters or previously entered
2194 commands. It specifies no action other than that the receiver send a
2195 "250 OK" reply.
2197 This command has no effect on the reverse-path buffer, the forward-
2198 path buffer, or the mail data buffer, and it may be issued at any
2199 time. If a parameter string is specified, servers SHOULD ignore it.
2201 Syntax:
2203 noop = "NOOP" [ SP String ] CRLF
2205 4.1.1.10. QUIT (QUIT)
2207 This command specifies that the receiver MUST send a "221 OK" reply,
2208 and then close the transmission channel.
2210 The receiver MUST NOT intentionally close the transmission channel
2211 until it receives and replies to a QUIT command (even if there was an
2212 error). The sender MUST NOT intentionally close the transmission
2213 channel until it sends a QUIT command, and it SHOULD wait until it
2214 receives the reply (even if there was an error response to a previous
2215 command). If the connection is closed prematurely due to violations
2216 of the above or system or network failure, the server MUST cancel any
2217 pending transaction, but not undo any previously completed
2218 transaction, and generally MUST act as if the command or transaction
2219 in progress had received a temporary error (i.e., a 4yz response).
2221 The QUIT command may be issued at any time. Any current uncompleted
2222 mail transaction will be aborted.
2224 Syntax:
2226 quit = "QUIT" CRLF
2228 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses
2230 If the server SMTP does not recognize or cannot implement one or more
2231 of the parameters associated with a particular MAIL FROM or RCPT TO
2232 command, it will return code 555.
2234 If, for some reason, the server is temporarily unable to accommodate
2235 one or more of the parameters associated with a MAIL FROM or RCPT TO
2236 command, and if the definition of the specific parameter does not
2237 mandate the use of another code, it should return code 455.
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2244 RFC 5321 SMTP October 2008
2247 Errors specific to particular parameters and their values will be
2248 specified in the parameter's defining RFC.
2250 4.1.2. Command Argument Syntax
2252 The syntax of the argument clauses of the above commands (using the
2253 syntax specified in RFC 5234 [7] where applicable) is given below.
2254 Some of the productions given below are used only in conjunction with
2255 source routes as described in Appendix C. Terminals not defined in
2256 this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined
2257 in the "core" syntax in Section 6 of RFC 5234 [7] or in the message
2258 format syntax in RFC 5322 [4].
2260 Reverse-path = Path / "<>"
2262 Forward-path = Path
2264 Path = "<" [ A-d-l ":" ] Mailbox ">"
2266 A-d-l = At-domain *( "," At-domain )
2267 ; Note that this form, the so-called "source
2268 ; route", MUST BE accepted, SHOULD NOT be
2269 ; generated, and SHOULD be ignored.
2271 At-domain = "@" Domain
2273 Mail-parameters = esmtp-param *(SP esmtp-param)
2275 Rcpt-parameters = esmtp-param *(SP esmtp-param)
2277 esmtp-param = esmtp-keyword ["=" esmtp-value]
2279 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
2281 esmtp-value = 1*(%d33-60 / %d62-126)
2282 ; any CHAR excluding "=", SP, and control
2283 ; characters. If this string is an email address,
2284 ; i.e., a Mailbox, then the "xtext" syntax [32]
2285 ; SHOULD be used.
2287 Keyword = Ldh-str
2289 Argument = Atom
2291 Domain = sub-domain *("." sub-domain)
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2300 RFC 5321 SMTP October 2008
2303 sub-domain = Let-dig [Ldh-str]
2305 Let-dig = ALPHA / DIGIT
2307 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
2309 address-literal = "[" ( IPv4-address-literal /
2310 IPv6-address-literal /
2311 General-address-literal ) "]"
2312 ; See Section 4.1.3
2314 Mailbox = Local-part "@" ( Domain / address-literal )
2316 Local-part = Dot-string / Quoted-string
2317 ; MAY be case-sensitive
2320 Dot-string = Atom *("." Atom)
2322 Atom = 1*atext
2324 Quoted-string = DQUOTE *QcontentSMTP DQUOTE
2326 QcontentSMTP = qtextSMTP / quoted-pairSMTP
2328 quoted-pairSMTP = %d92 %d32-126
2329 ; i.e., backslash followed by any ASCII
2330 ; graphic (including itself) or SPace
2332 qtextSMTP = %d32-33 / %d35-91 / %d93-126
2333 ; i.e., within a quoted string, any
2334 ; ASCII graphic or space is permitted
2335 ; without blackslash-quoting except
2336 ; double-quote and the backslash itself.
2338 String = Atom / Quoted-string
2340 While the above definition for Local-part is relatively permissive,
2341 for maximum interoperability, a host that expects to receive mail
2342 SHOULD avoid defining mailboxes where the Local-part requires (or
2343 uses) the Quoted-string form or where the Local-part is case-
2344 sensitive. For any purposes that require generating or comparing
2345 Local-parts (e.g., to specific mailbox names), all quoted forms MUST
2346 be treated as equivalent, and the sending system SHOULD transmit the
2347 form that uses the minimum quoting possible.
2349 Systems MUST NOT define mailboxes in such a way as to require the use
2350 in SMTP of non-ASCII characters (octets with the high order bit set
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2356 RFC 5321 SMTP October 2008
2359 to one) or ASCII "control characters" (decimal value 0-31 and 127).
2360 These characters MUST NOT be used in MAIL or RCPT commands or other
2361 commands that require mailbox names.
2363 Note that the backslash, "\", is a quote character, which is used to
2364 indicate that the next character is to be used literally (instead of
2365 its normal interpretation). For example, "Joe\,Smith" indicates a
2366 single nine-character user name string with the comma being the
2367 fourth character of that string.
2369 To promote interoperability and consistent with long-standing
2370 guidance about conservative use of the DNS in naming and applications
2371 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
2372 characters outside the set of alphabetic characters, digits, and
2373 hyphen MUST NOT appear in domain name labels for SMTP clients or
2374 servers. In particular, the underscore character is not permitted.
2375 SMTP servers that receive a command in which invalid character codes
2376 have been employed, and for which there are no other reasons for
2377 rejection, MUST reject that command with a 501 response (this rule,
2378 like others, could be overridden by appropriate SMTP extensions).
2380 4.1.3. Address Literals
2382 Sometimes a host is not known to the domain name system and
2383 communication (and, in particular, communication to report and repair
2384 the error) is blocked. To bypass this barrier, a special literal
2385 form of the address is allowed as an alternative to a domain name.
2386 For IPv4 addresses, this form uses four small decimal integers
2387 separated by dots and enclosed by brackets such as [123.255.37.2],
2388 which indicates an (IPv4) Internet Address in sequence-of-octets
2389 form. For IPv6 and other forms of addressing that might eventually
2390 be standardized, the form consists of a standardized "tag" that
2391 identifies the address syntax, a colon, and the address itself, in a
2392 format specified as part of the relevant standards (i.e., RFC 4291
2393 [8] for IPv6).
2395 Specifically:
2397 IPv4-address-literal = Snum 3("." Snum)
2399 IPv6-address-literal = "IPv6:" IPv6-addr
2401 General-address-literal = Standardized-tag ":" 1*dcontent
2403 Standardized-tag = Ldh-str
2404 ; Standardized-tag MUST be specified in a
2405 ; Standards-Track RFC and registered with IANA
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2412 RFC 5321 SMTP October 2008
2415 dcontent = %d33-90 / ; Printable US-ASCII
2416 %d94-126 ; excl. "[", "\", "]"
2418 Snum = 1*3DIGIT
2419 ; representing a decimal integer
2420 ; value in the range 0 through 255
2422 IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
2424 IPv6-hex = 1*4HEXDIG
2426 IPv6-full = IPv6-hex 7(":" IPv6-hex)
2428 IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::"
2429 [IPv6-hex *5(":" IPv6-hex)]
2430 ; The "::" represents at least 2 16-bit groups of
2431 ; zeros. No more than 6 groups in addition to the
2432 ; "::" may be present.
2434 IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
2436 IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"
2437 [IPv6-hex *3(":" IPv6-hex) ":"]
2438 IPv4-address-literal
2439 ; The "::" represents at least 2 16-bit groups of
2440 ; zeros. No more than 4 groups in addition to the
2441 ; "::" and IPv4-address-literal may be present.
2443 4.1.4. Order of Commands
2445 There are restrictions on the order in which these commands may be
2446 used.
2448 A session that will contain mail transactions MUST first be
2449 initialized by the use of the EHLO command. An SMTP server SHOULD
2450 accept commands for non-mail transactions (e.g., VRFY or EXPN)
2451 without this initialization.
2453 An EHLO command MAY be issued by a client later in the session. If
2454 it is issued after the session begins and the EHLO command is
2455 acceptable to the SMTP server, the SMTP server MUST clear all buffers
2456 and reset the state exactly as if a RSET command had been issued. In
2457 other words, the sequence of RSET followed immediately by EHLO is
2458 redundant, but not harmful other than in the performance cost of
2459 executing unnecessary commands.
2461 If the EHLO command is not acceptable to the SMTP server, 501, 500,
2462 502, or 550 failure replies MUST be returned as appropriate. The
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2468 RFC 5321 SMTP October 2008
2471 SMTP server MUST stay in the same state after transmitting these
2472 replies that it was in before the EHLO was received.
2474 The SMTP client MUST, if possible, ensure that the domain parameter
2475 to the EHLO command is a primary host name as specified for this
2476 command in Section 2.3.5. If this is not possible (e.g., when the
2477 client's address is dynamically assigned and the client does not have
2478 an obvious name), an address literal SHOULD be substituted for the
2479 domain name.
2481 An SMTP server MAY verify that the domain name argument in the EHLO
2482 command actually corresponds to the IP address of the client.
2483 However, if the verification fails, the server MUST NOT refuse to
2484 accept a message on that basis. Information captured in the
2485 verification attempt is for logging and tracing purposes. Note that
2486 this prohibition applies to the matching of the parameter to its IP
2487 address only; see Section 7.9 for a more extensive discussion of
2488 rejecting incoming connections or mail messages.
2490 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
2491 during a session, or without previously initializing a session. SMTP
2492 servers SHOULD process these normally (that is, not return a 503
2493 code) even if no EHLO command has yet been received; clients SHOULD
2494 open a session with EHLO before sending these commands.
2496 If these rules are followed, the example in RFC 821 that shows "550
2497 access denied to you" in response to an EXPN command is incorrect
2498 unless an EHLO command precedes the EXPN or the denial of access is
2499 based on the client's IP address or other authentication or
2500 authorization-determining mechanisms.
2502 The MAIL command (or the obsolete SEND, SOML, or SAML commands)
2503 begins a mail transaction. Once started, a mail transaction consists
2504 of a transaction beginning command, one or more RCPT commands, and a
2505 DATA command, in that order. A mail transaction may be aborted by
2506 the RSET, a new EHLO, or the QUIT command. There may be zero or more
2507 transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be
2508 sent if a mail transaction is already open, i.e., it should be sent
2509 only if no mail transaction had been started in the session, or if
2510 the previous one successfully concluded with a successful DATA
2511 command, or if the previous one was aborted, e.g., with a RSET or new
2512 EHLO.
2514 If the transaction beginning command argument is not acceptable, a
2515 501 failure reply MUST be returned and the SMTP server MUST stay in
2516 the same state. If the commands in a transaction are out of order to
2517 the degree that they cannot be processed by the server, a 503 failure
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2524 RFC 5321 SMTP October 2008
2527 reply MUST be returned and the SMTP server MUST stay in the same
2528 state.
2530 The last command in a session MUST be the QUIT command. The QUIT
2531 command SHOULD be used by the client SMTP to request connection
2532 closure, even when no session opening command was sent and accepted.
2534 4.1.5. Private-Use Commands
2536 As specified in Section 2.2.2, commands starting in "X" may be used
2537 by bilateral agreement between the client (sending) and server
2538 (receiving) SMTP agents. An SMTP server that does not recognize such
2539 a command is expected to reply with "500 Command not recognized". An
2540 extended SMTP server MAY list the feature names associated with these
2541 private commands in the response to the EHLO command.
2543 Commands sent or accepted by SMTP systems that do not start with "X"
2544 MUST conform to the requirements of Section 2.2.2.
2546 4.2. SMTP Replies
2548 Replies to SMTP commands serve to ensure the synchronization of
2549 requests and actions in the process of mail transfer and to guarantee
2550 that the SMTP client always knows the state of the SMTP server.
2551 Every command MUST generate exactly one reply.
2553 The details of the command-reply sequence are described in
2554 Section 4.3.
2556 An SMTP reply consists of a three digit number (transmitted as three
2557 numeric characters) followed by some text unless specified otherwise
2558 in this document. The number is for use by automata to determine
2559 what state to enter next; the text is for the human user. The three
2560 digits contain enough encoded information that the SMTP client need
2561 not examine the text and may either discard it or pass it on to the
2562 user, as appropriate. Exceptions are as noted elsewhere in this
2563 document. In particular, the 220, 221, 251, 421, and 551 reply codes
2564 are associated with message text that must be parsed and interpreted
2565 by machines. In the general case, the text may be receiver dependent
2566 and context dependent, so there are likely to be varying texts for
2567 each reply code. A discussion of the theory of reply codes is given
2568 in Section 4.2.1. Formally, a reply is defined to be the sequence: a
2569 three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
2570 reply (as defined in the same section). Since, in violation of this
2571 specification, the text is sometimes not sent, clients that do not
2572 receive it SHOULD be prepared to process the code alone (with or
2573 without a trailing space character). Only the EHLO, EXPN, and HELP
2574 commands are expected to result in multiline replies in normal
2578 Klensin Standards Track [Page 46]
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2580 RFC 5321 SMTP October 2008
2583 circumstances; however, multiline replies are allowed for any
2584 command.
2586 In ABNF, server responses are:
2588 Greeting = ( "220 " (Domain / address-literal)
2589 [ SP textstring ] CRLF ) /
2590 ( "220-" (Domain / address-literal)
2591 [ SP textstring ] CRLF
2592 *( "220-" [ textstring ] CRLF )
2593 "220" [ SP textstring ] CRLF )
2595 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII
2597 Reply-line = *( Reply-code "-" [ textstring ] CRLF )
2598 Reply-code [ SP textstring ] CRLF
2600 Reply-code = %x32-35 %x30-35 %x30-39
2602 where "Greeting" appears only in the 220 response that announces that
2603 the server is opening its part of the connection. (Other possible
2604 server responses upon connection follow the syntax of Reply-line.)
2606 An SMTP server SHOULD send only the reply codes listed in this
2607 document. An SMTP server SHOULD use the text shown in the examples
2608 whenever appropriate.
2610 An SMTP client MUST determine its actions only by the reply code, not
2611 by the text (except for the "change of address" 251 and 551 and, if
2612 necessary, 220, 221, and 421 replies); in the general case, any text,
2613 including no text at all (although senders SHOULD NOT send bare
2614 codes), MUST be acceptable. The space (blank) following the reply
2615 code is considered part of the text. Whenever possible, a receiver-
2616 SMTP SHOULD test the first digit (severity indication) of the reply
2617 code.
2619 The list of codes that appears below MUST NOT be construed as
2620 permanent. While the addition of new codes should be a rare and
2621 significant activity, with supplemental information in the textual
2622 part of the response being preferred, new codes may be added as the
2623 result of new Standards or Standards-Track specifications.
2624 Consequently, a sender-SMTP MUST be prepared to handle codes not
2625 specified in this document and MUST do so by interpreting the first
2626 digit only.
2628 In the absence of extensions negotiated with the client, SMTP servers
2629 MUST NOT send reply codes whose first digits are other than 2, 3, 4,
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2636 RFC 5321 SMTP October 2008
2639 or 5. Clients that receive such out-of-range codes SHOULD normally
2640 treat them as fatal errors and terminate the mail transaction.
2642 4.2.1. Reply Code Severities and Theory
2644 The three digits of the reply each have a special significance. The
2645 first digit denotes whether the response is good, bad, or incomplete.
2646 An unsophisticated SMTP client, or one that receives an unexpected
2647 code, will be able to determine its next action (proceed as planned,
2648 redo, retrench, etc.) by examining this first digit. An SMTP client
2649 that wants to know approximately what kind of error occurred (e.g.,
2650 mail system error, command syntax error) may examine the second
2651 digit. The third digit and any supplemental information that may be
2652 present is reserved for the finest gradation of information.
2654 There are four values for the first digit of the reply code:
2656 2yz Positive Completion reply
2657 The requested action has been successfully completed. A new
2658 request may be initiated.
2660 3yz Positive Intermediate reply
2661 The command has been accepted, but the requested action is being
2662 held in abeyance, pending receipt of further information. The
2663 SMTP client should send another command specifying this
2664 information. This reply is used in command sequence groups (i.e.,
2665 in DATA).
2667 4yz Transient Negative Completion reply
2668 The command was not accepted, and the requested action did not
2669 occur. However, the error condition is temporary, and the action
2670 may be requested again. The sender should return to the beginning
2671 of the command sequence (if any). It is difficult to assign a
2672 meaning to "transient" when two different sites (receiver- and
2673 sender-SMTP agents) must agree on the interpretation. Each reply
2674 in this category might have a different time value, but the SMTP
2675 client SHOULD try again. A rule of thumb to determine whether a
2676 reply fits into the 4yz or the 5yz category (see below) is that
2677 replies are 4yz if they can be successful if repeated without any
2678 change in command form or in properties of the sender or receiver
2679 (that is, the command is repeated identically and the receiver
2680 does not put up a new implementation).
2682 5yz Permanent Negative Completion reply
2683 The command was not accepted and the requested action did not
2684 occur. The SMTP client SHOULD NOT repeat the exact request (in
2685 the same sequence). Even some "permanent" error conditions can be
2686 corrected, so the human user may want to direct the SMTP client to
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2692 RFC 5321 SMTP October 2008
2695 reinitiate the command sequence by direct action at some point in
2696 the future (e.g., after the spelling has been changed, or the user
2697 has altered the account status).
2699 It is worth noting that the file transfer protocol (FTP) [34] uses a
2700 very similar code architecture and that the SMTP codes are based on
2701 the FTP model. However, SMTP uses a one-command, one-response model
2702 (while FTP is asynchronous) and FTP's 1yz codes are not part of the
2703 SMTP model.
2705 The second digit encodes responses in specific categories:
2707 x0z Syntax: These replies refer to syntax errors, syntactically
2708 correct commands that do not fit any functional category, and
2709 unimplemented or superfluous commands.
2711 x1z Information: These are replies to requests for information, such
2712 as status or help.
2714 x2z Connections: These are replies referring to the transmission
2715 channel.
2717 x3z Unspecified.
2719 x4z Unspecified.
2721 x5z Mail system: These replies indicate the status of the receiver
2722 mail system vis-a-vis the requested transfer or other mail system
2723 action.
2725 The third digit gives a finer gradation of meaning in each category
2726 specified by the second digit. The list of replies illustrates this.
2727 Each reply text is recommended rather than mandatory, and may even
2728 change according to the command with which it is associated. On the
2729 other hand, the reply codes must strictly follow the specifications
2730 in this section. Receiver implementations should not invent new
2731 codes for slightly different situations from the ones described here,
2732 but rather adapt codes already defined.
2734 For example, a command such as NOOP, whose successful execution does
2735 not offer the SMTP client any new information, will return a 250
2736 reply. The reply is 502 when the command requests an unimplemented
2737 non-site-specific action. A refinement of that is the 504 reply for
2738 a command that is implemented, but that requests an unimplemented
2739 parameter.
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2748 RFC 5321 SMTP October 2008
2751 The reply text may be longer than a single line; in these cases the
2752 complete text must be marked so the SMTP client knows when it can
2753 stop reading the reply. This requires a special format to indicate a
2754 multiple line reply.
2756 The format for multiline replies requires that every line, except the
2757 last, begin with the reply code, followed immediately by a hyphen,
2758 "-" (also known as minus), followed by text. The last line will
2759 begin with the reply code, followed immediately by <SP>, optionally
2760 some text, and <CRLF>. As noted above, servers SHOULD send the <SP>
2761 if subsequent text is not sent, but clients MUST be prepared for it
2762 to be omitted.
2764 For example:
2766 250-First line
2767 250-Second line
2768 250-234 Text beginning with numbers
2769 250 The last line
2771 In a multiline reply, the reply code on each of the lines MUST be the
2772 same. It is reasonable for the client to rely on this, so it can
2773 make processing decisions based on the code in any line, assuming
2774 that all others will be the same. In a few cases, there is important
2775 data for the client in the reply "text". The client will be able to
2776 identify these cases from the current context.
2778 4.2.2. Reply Codes by Function Groups
2780 500 Syntax error, command unrecognized (This may include errors such
2781 as command line too long)
2783 501 Syntax error in parameters or arguments
2785 502 Command not implemented (see Section 4.2.4)
2787 503 Bad sequence of commands
2789 504 Command parameter not implemented
2792 211 System status, or system help reply
2794 214 Help message (Information on how to use the receiver or the
2795 meaning of a particular non-standard command; this reply is useful
2796 only to the human user)
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2804 RFC 5321 SMTP October 2008
2807 220 <domain> Service ready
2809 221 <domain> Service closing transmission channel
2811 421 <domain> Service not available, closing transmission channel
2812 (This may be a reply to any command if the service knows it must
2813 shut down)
2816 250 Requested mail action okay, completed
2818 251 User not local; will forward to <forward-path> (See Section 3.4)
2820 252 Cannot VRFY user, but will accept message and attempt delivery
2821 (See Section 3.5.3)
2823 455 Server unable to accommodate parameters
2825 555 MAIL FROM/RCPT TO parameters not recognized or not implemented
2827 450 Requested mail action not taken: mailbox unavailable (e.g.,
2828 mailbox busy or temporarily blocked for policy reasons)
2830 550 Requested action not taken: mailbox unavailable (e.g., mailbox
2831 not found, no access, or command rejected for policy reasons)
2833 451 Requested action aborted: error in processing
2835 551 User not local; please try <forward-path> (See Section 3.4)
2837 452 Requested action not taken: insufficient system storage
2839 552 Requested mail action aborted: exceeded storage allocation
2841 553 Requested action not taken: mailbox name not allowed (e.g.,
2842 mailbox syntax incorrect)
2844 354 Start mail input; end with <CRLF>.<CRLF>
2846 554 Transaction failed (Or, in the case of a connection-opening
2847 response, "No SMTP service here")
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2860 RFC 5321 SMTP October 2008
2863 4.2.3. Reply Codes in Numeric Order
2865 211 System status, or system help reply
2867 214 Help message (Information on how to use the receiver or the
2868 meaning of a particular non-standard command; this reply is useful
2869 only to the human user)
2871 220 <domain> Service ready
2873 221 <domain> Service closing transmission channel
2875 250 Requested mail action okay, completed
2877 251 User not local; will forward to <forward-path> (See Section 3.4)
2879 252 Cannot VRFY user, but will accept message and attempt delivery
2880 (See Section 3.5.3)
2882 354 Start mail input; end with <CRLF>.<CRLF>
2884 421 <domain> Service not available, closing transmission channel
2885 (This may be a reply to any command if the service knows it must
2886 shut down)
2888 450 Requested mail action not taken: mailbox unavailable (e.g.,
2889 mailbox busy or temporarily blocked for policy reasons)
2891 451 Requested action aborted: local error in processing
2893 452 Requested action not taken: insufficient system storage
2895 455 Server unable to accommodate parameters
2897 500 Syntax error, command unrecognized (This may include errors such
2898 as command line too long)
2900 501 Syntax error in parameters or arguments
2902 502 Command not implemented (see Section 4.2.4)
2904 503 Bad sequence of commands
2906 504 Command parameter not implemented
2908 550 Requested action not taken: mailbox unavailable (e.g., mailbox
2909 not found, no access, or command rejected for policy reasons)
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2916 RFC 5321 SMTP October 2008
2919 551 User not local; please try <forward-path> (See Section 3.4)
2921 552 Requested mail action aborted: exceeded storage allocation
2923 553 Requested action not taken: mailbox name not allowed (e.g.,
2924 mailbox syntax incorrect)
2926 554 Transaction failed (Or, in the case of a connection-opening
2927 response, "No SMTP service here")
2929 555 MAIL FROM/RCPT TO parameters not recognized or not implemented
2931 4.2.4. Reply Code 502
2933 Questions have been raised as to when reply code 502 (Command not
2934 implemented) SHOULD be returned in preference to other codes. 502
2935 SHOULD be used when the command is actually recognized by the SMTP
2936 server, but not implemented. If the command is not recognized, code
2937 500 SHOULD be returned. Extended SMTP systems MUST NOT list
2938 capabilities in response to EHLO for which they will return 502 (or
2939 500) replies.
2941 4.2.5. Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>
2943 When an SMTP server returns a positive completion status (2yz code)
2944 after the DATA command is completed with <CRLF>.<CRLF>, it accepts
2945 responsibility for:
2947 o delivering the message (if the recipient mailbox exists), or
2949 o if attempts to deliver the message fail due to transient
2950 conditions, retrying delivery some reasonable number of times at
2951 intervals as specified in Section 4.5.4.
2953 o if attempts to deliver the message fail due to permanent
2954 conditions, or if repeated attempts to deliver the message fail
2955 due to transient conditions, returning appropriate notification to
2956 the sender of the original message (using the address in the SMTP
2957 MAIL command).
2959 When an SMTP server returns a temporary error status (4yz) code after
2960 the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make a
2961 subsequent attempt to deliver that message. The SMTP client retains
2962 responsibility for the delivery of that message and may either return
2963 it to the user or requeue it for a subsequent attempt (see
2964 Section 4.5.4.1).
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2972 RFC 5321 SMTP October 2008
2975 The user who originated the message SHOULD be able to interpret the
2976 return of a transient failure status (by mail message or otherwise)
2977 as a non-delivery indication, just as a permanent failure would be
2978 interpreted. If the client SMTP successfully handles these
2979 conditions, the user will not receive such a reply.
2981 When an SMTP server returns a permanent error status (5yz) code after
2982 the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
2983 any subsequent attempt to deliver the message. As with temporary
2984 error status codes, the SMTP client retains responsibility for the
2985 message, but SHOULD not again attempt delivery to the same server
2986 without user review of the message and response and appropriate
2987 intervention.
2989 4.3. Sequencing of Commands and Replies
2991 4.3.1. Sequencing Overview
2993 The communication between the sender and receiver is an alternating
2994 dialogue, controlled by the sender. As such, the sender issues a
2995 command and the receiver responds with a reply. Unless other
2996 arrangements are negotiated through service extensions, the sender
2997 MUST wait for this response before sending further commands. One
2998 important reply is the connection greeting. Normally, a receiver
2999 will send a 220 "Service ready" reply when the connection is
3000 completed. The sender SHOULD wait for this greeting message before
3001 sending any commands.
3003 Note: all the greeting-type replies have the official name (the
3004 fully-qualified primary domain name) of the server host as the first
3005 word following the reply code. Sometimes the host will have no
3006 meaningful name. See Section 4.1.3 for a discussion of alternatives
3007 in these situations.
3009 For example,
3011 220 ISIF.USC.EDU Service ready
3013 or
3015 220 mail.example.com SuperSMTP v 6.1.2 Service ready
3017 or
3019 220 [10.0.0.1] Clueless host service ready
3021 The table below lists alternative success and failure replies for
3022 each command. These SHOULD be strictly adhered to. A receiver MAY
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3028 RFC 5321 SMTP October 2008
3031 substitute text in the replies, but the meanings and actions implied
3032 by the code numbers and by the specific command reply sequence MUST
3033 be preserved.
3035 4.3.2. Command-Reply Sequences
3037 Each command is listed with its usual possible replies. The prefixes
3038 used before the possible replies are "I" for intermediate, "S" for
3039 success, and "E" for error. Since some servers may generate other
3040 replies under special circumstances, and to allow for future
3041 extension, SMTP clients SHOULD, when possible, interpret only the
3042 first digit of the reply and MUST be prepared to deal with
3043 unrecognized reply codes by interpreting the first digit only.
3044 Unless extended using the mechanisms described in Section 2.2, SMTP
3045 servers MUST NOT transmit reply codes to an SMTP client that are
3046 other than three digits or that do not start in a digit between 2 and
3047 5 inclusive.
3049 These sequencing rules and, in principle, the codes themselves, can
3050 be extended or modified by SMTP extensions offered by the server and
3051 accepted (requested) by the client. However, if the target is more
3052 precise granularity in the codes, rather than codes for completely
3053 new purposes, the system described in RFC 3463 [25] SHOULD be used in
3054 preference to the invention of new codes.
3056 In addition to the codes listed below, any SMTP command can return
3057 any of the following codes if the corresponding unusual circumstances
3058 are encountered:
3060 500 For the "command line too long" case or if the command name was
3061 not recognized. Note that producing a "command not recognized"
3062 error in response to the required subset of these commands is a
3063 violation of this specification. Similarly, producing a "command
3064 too long" message for a command line shorter than 512 characters
3065 would violate the provisions of Section 4.5.3.1.4.
3067 501 Syntax error in command or arguments. In order to provide for
3068 future extensions, commands that are specified in this document as
3069 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
3070 message if arguments are supplied in the absence of EHLO-
3071 advertised extensions.
3073 421 Service shutting down and closing transmission channel
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3084 RFC 5321 SMTP October 2008
3087 Specific sequences are:
3089 CONNECTION ESTABLISHMENT
3091 S: 220
3092 E: 554
3094 EHLO or HELO
3096 S: 250
3097 E: 504 (a conforming implementation could return this code only
3098 in fairly obscure cases), 550, 502 (permitted only with an old-
3099 style server that does not support EHLO)
3101 MAIL
3103 S: 250
3104 E: 552, 451, 452, 550, 553, 503, 455, 555
3106 RCPT
3108 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551)
3109 E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555
3111 DATA
3113 I: 354 -> data -> S: 250
3115 E: 552, 554, 451, 452
3117 E: 450, 550 (rejections for policy reasons)
3119 E: 503, 554
3121 RSET
3123 S: 250
3125 VRFY
3127 S: 250, 251, 252
3128 E: 550, 551, 553, 502, 504
3130 EXPN
3132 S: 250, 252
3133 E: 550, 500, 502, 504
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3140 RFC 5321 SMTP October 2008
3143 HELP
3145 S: 211, 214
3146 E: 502, 504
3148 NOOP
3150 S: 250
3152 QUIT
3154 S: 221
3156 4.4. Trace Information
3158 When an SMTP server receives a message for delivery or further
3159 processing, it MUST insert trace ("time stamp" or "Received")
3160 information at the beginning of the message content, as discussed in
3161 Section 4.1.1.4.
3163 This line MUST be structured as follows:
3165 o The FROM clause, which MUST be supplied in an SMTP environment,
3166 SHOULD contain both (1) the name of the source host as presented
3167 in the EHLO command and (2) an address literal containing the IP
3168 address of the source, determined from the TCP connection.
3170 o The ID clause MAY contain an "@" as suggested in RFC 822, but this
3171 is not required.
3173 o If the FOR clause appears, it MUST contain exactly one <path>
3174 entry, even when multiple RCPT commands have been given. Multiple
3175 <path>s raise some security issues and have been deprecated, see
3176 Section 7.2.
3178 An Internet mail program MUST NOT change or delete a Received: line
3179 that was previously added to the message header section. SMTP
3180 servers MUST prepend Received lines to messages; they MUST NOT change
3181 the order of existing lines or insert Received lines in any other
3182 location.
3184 As the Internet grows, comparability of Received header fields is
3185 important for detecting problems, especially slow relays. SMTP
3186 servers that create Received header fields SHOULD use explicit
3187 offsets in the dates (e.g., -0800), rather than time zone names of
3188 any type. Local time (with an offset) SHOULD be used rather than UT
3189 when feasible. This formulation allows slightly more information
3190 about local circumstances to be specified. If UT is needed, the
3194 Klensin Standards Track [Page 57]
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3196 RFC 5321 SMTP October 2008
3199 receiver need merely do some simple arithmetic to convert the values.
3200 Use of UT loses information about the time zone-location of the
3201 server. If it is desired to supply a time zone name, it SHOULD be
3202 included in a comment.
3204 When the delivery SMTP server makes the "final delivery" of a
3205 message, it inserts a return-path line at the beginning of the mail
3206 data. This use of return-path is required; mail systems MUST support
3207 it. The return-path line preserves the information in the <reverse-
3208 path> from the MAIL command. Here, final delivery means the message
3209 has left the SMTP environment. Normally, this would mean it had been
3210 delivered to the destination user or an associated mail drop, but in
3211 some cases it may be further processed and transmitted by another
3212 mail system.
3214 It is possible for the mailbox in the return path to be different
3215 from the actual sender's mailbox, for example, if error responses are
3216 to be delivered to a special error handling mailbox rather than to
3217 the message sender. When mailing lists are involved, this
3218 arrangement is common and useful as a means of directing errors to
3219 the list maintainer rather than the message originator.
3221 The text above implies that the final mail data will begin with a
3222 return path line, followed by one or more time stamp lines. These
3223 lines will be followed by the rest of the mail data: first the
3224 balance of the mail header section and then the body (RFC 5322 [4]).
3226 It is sometimes difficult for an SMTP server to determine whether or
3227 not it is making final delivery since forwarding or other operations
3228 may occur after the message is accepted for delivery. Consequently,
3229 any further (forwarding, gateway, or relay) systems MAY remove the
3230 return path and rebuild the MAIL command as needed to ensure that
3231 exactly one such line appears in a delivered message.
3233 A message-originating SMTP system SHOULD NOT send a message that
3234 already contains a Return-path header field. SMTP servers performing
3235 a relay function MUST NOT inspect the message data, and especially
3236 not to the extent needed to determine if Return-path header fields
3237 are present. SMTP servers making final delivery MAY remove Return-
3238 path header fields before adding their own.
3240 The primary purpose of the Return-path is to designate the address to
3241 which messages indicating non-delivery or other mail system failures
3242 are to be sent. For this to be unambiguous, exactly one return path
3243 SHOULD be present when the message is delivered. Systems using RFC
3244 822 syntax with non-SMTP transports SHOULD designate an unambiguous
3245 address, associated with the transport envelope, to which error
3246 reports (e.g., non-delivery messages) should be sent.
3250 Klensin Standards Track [Page 58]
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3252 RFC 5321 SMTP October 2008
3255 Historical note: Text in RFC 822 that appears to contradict the use
3256 of the Return-path header field (or the envelope reverse-path address
3257 from the MAIL command) as the destination for error messages is not
3258 applicable on the Internet. The reverse-path address (as copied into
3259 the Return-path) MUST be used as the target of any mail containing
3260 delivery error messages.
3262 In particular:
3263 o a gateway from SMTP -> elsewhere SHOULD insert a return-path
3264 header field, unless it is known that the "elsewhere" transport
3265 also uses Internet domain addresses and maintains the envelope
3266 sender address separately.
3268 o a gateway from elsewhere -> SMTP SHOULD delete any return-path
3269 header field present in the message, and either copy that
3270 information to the SMTP envelope or combine it with information
3271 present in the envelope of the other transport system to construct
3272 the reverse-path argument to the MAIL command in the SMTP
3273 envelope.
3275 The server must give special treatment to cases in which the
3276 processing following the end of mail data indication is only
3277 partially successful. This could happen if, after accepting several
3278 recipients and the mail data, the SMTP server finds that the mail
3279 data could be successfully delivered to some, but not all, of the
3280 recipients. In such cases, the response to the DATA command MUST be
3281 an OK reply. However, the SMTP server MUST compose and send an
3282 "undeliverable mail" notification message to the originator of the
3283 message.
3285 A single notification listing all of the failed recipients or
3286 separate notification messages MUST be sent for each failed
3287 recipient. For economy of processing by the sender, the former
3288 SHOULD be used when possible. Note that the key difference between
3289 handling aliases (Section 3.9.1) and forwarding (this subsection) is
3290 the change to the backward-pointing address in this case. All
3291 notification messages about undeliverable mail MUST be sent using the
3292 MAIL command (even if they result from processing the obsolete SEND,
3293 SOML, or SAML commands) and MUST use a null return path as discussed
3294 in Section 3.6.
3296 The time stamp line and the return path line are formally defined as
3297 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322
3298 [4]):
3300 Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
3302 Time-stamp-line = "Received:" FWS Stamp <CRLF>
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3307 \f
3308 RFC 5321 SMTP October 2008
3311 Stamp = From-domain By-domain Opt-info [CFWS] ";"
3312 FWS date-time
3313 ; where "date-time" is as defined in RFC 5322 [4]
3314 ; but the "obs-" forms, especially two-digit
3315 ; years, are prohibited in SMTP and MUST NOT be used.
3317 From-domain = "FROM" FWS Extended-Domain
3319 By-domain = CFWS "BY" FWS Extended-Domain
3321 Extended-Domain = Domain /
3322 ( Domain FWS "(" TCP-info ")" ) /
3323 ( address-literal FWS "(" TCP-info ")" )
3325 TCP-info = address-literal / ( Domain FWS address-literal )
3326 ; Information derived by server from TCP connection
3327 ; not client EHLO.
3329 Opt-info = [Via] [With] [ID] [For]
3330 [Additional-Registered-Clauses]
3332 Via = CFWS "VIA" FWS Link
3334 With = CFWS "WITH" FWS Protocol
3336 ID = CFWS "ID" FWS ( Atom / msg-id )
3337 ; msg-id is defined in RFC 5322 [4]
3339 For = CFWS "FOR" FWS ( Path / Mailbox )
3341 Additional-Registered-Clauses = CFWS Atom FWS String
3342 ; Additional standard clauses may be
3343 added in this
3344 ; location by future standards and
3345 registration with
3346 ; IANA. SMTP servers SHOULD NOT use
3347 unregistered
3348 ; names. See Section 8.
3350 Link = "TCP" / Addtl-Link
3352 Addtl-Link = Atom
3353 ; Additional standard names for links are
3354 ; registered with the Internet Assigned Numbers
3355 ; Authority (IANA). "Via" is primarily of value
3356 ; with non-Internet transports. SMTP servers
3357 ; SHOULD NOT use unregistered names.
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3364 RFC 5321 SMTP October 2008
3367 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
3369 Attdl-Protocol = Atom
3370 ; Additional standard names for protocols are
3371 ; registered with the Internet Assigned Numbers
3372 ; Authority (IANA) in the "mail parameters"
3373 ; registry [9]. SMTP servers SHOULD NOT
3374 ; use unregistered names.
3376 4.5. Additional Implementation Issues
3378 4.5.1. Minimum Implementation
3380 In order to make SMTP workable, the following minimum implementation
3381 MUST be provided by all receivers. The following commands MUST be
3382 supported to conform to this specification:
3384 EHLO
3385 HELO
3386 MAIL
3387 RCPT
3388 DATA
3389 RSET
3390 NOOP
3391 QUIT
3392 VRFY
3394 Any system that includes an SMTP server supporting mail relaying or
3395 delivery MUST support the reserved mailbox "postmaster" as a case-
3396 insensitive local name. This postmaster address is not strictly
3397 necessary if the server always returns 554 on connection opening (as
3398 described in Section 3.1). The requirement to accept mail for
3399 postmaster implies that RCPT commands that specify a mailbox for
3400 postmaster at any of the domains for which the SMTP server provides
3401 mail service, as well as the special case of "RCPT TO:<Postmaster>"
3402 (with no domain specification), MUST be supported.
3404 SMTP systems are expected to make every reasonable effort to accept
3405 mail directed to Postmaster from any other system on the Internet.
3406 In extreme cases -- such as to contain a denial of service attack or
3407 other breach of security -- an SMTP server may block mail directed to
3408 Postmaster. However, such arrangements SHOULD be narrowly tailored
3409 so as to avoid blocking messages that are not part of such attacks.
3418 Klensin Standards Track [Page 61]
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3420 RFC 5321 SMTP October 2008
3423 4.5.2. Transparency
3425 Without some provision for data transparency, the character sequence
3426 "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
3427 In general, users are not aware of such "forbidden" sequences. To
3428 allow all user composed text to be transmitted transparently, the
3429 following procedures are used:
3431 o Before sending a line of mail text, the SMTP client checks the
3432 first character of the line. If it is a period, one additional
3433 period is inserted at the beginning of the line.
3435 o When a line of mail text is received by the SMTP server, it checks
3436 the line. If the line is composed of a single period, it is
3437 treated as the end of mail indicator. If the first character is a
3438 period and there are other characters on the line, the first
3439 character is deleted.
3441 The mail data may contain any of the 128 ASCII characters. All
3442 characters are to be delivered to the recipient's mailbox, including
3443 spaces, vertical and horizontal tabs, and other control characters.
3444 If the transmission channel provides an 8-bit byte (octet) data
3445 stream, the 7-bit ASCII codes are transmitted, right justified, in
3446 the octets, with the high-order bits cleared to zero. See
3447 Section 3.6 for special treatment of these conditions in SMTP systems
3448 serving a relay function.
3450 In some systems, it may be necessary to transform the data as it is
3451 received and stored. This may be necessary for hosts that use a
3452 different character set than ASCII as their local character set, that
3453 store data in records rather than strings, or which use special
3454 character sequences as delimiters inside mailboxes. If such
3455 transformations are necessary, they MUST be reversible, especially if
3456 they are applied to mail being relayed.
3458 4.5.3. Sizes and Timeouts
3460 4.5.3.1. Size Limits and Minimums
3462 There are several objects that have required minimum/maximum sizes.
3463 Every implementation MUST be able to receive objects of at least
3464 these sizes. Objects larger than these sizes SHOULD be avoided when
3465 possible. However, some Internet mail constructs such as encoded
3466 X.400 addresses (RFC 2156 [35]) will often require larger objects.
3467 Clients MAY attempt to transmit these, but MUST be prepared for a
3468 server to reject them if they cannot be handled by it. To the
3469 maximum extent possible, implementation techniques that impose no
3470 limits on the length of these objects should be used.
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3476 RFC 5321 SMTP October 2008
3479 Extensions to SMTP may involve the use of characters that occupy more
3480 than a single octet each. This section therefore specifies lengths
3481 in octets where absolute lengths, rather than character counts, are
3482 intended.
3484 4.5.3.1.1. Local-part
3486 The maximum total length of a user name or other local-part is 64
3487 octets.
3489 4.5.3.1.2. Domain
3491 The maximum total length of a domain name or number is 255 octets.
3493 4.5.3.1.3. Path
3495 The maximum total length of a reverse-path or forward-path is 256
3496 octets (including the punctuation and element separators).
3498 4.5.3.1.4. Command Line
3500 The maximum total length of a command line including the command word
3501 and the <CRLF> is 512 octets. SMTP extensions may be used to
3502 increase this limit.
3504 4.5.3.1.5. Reply Line
3506 The maximum total length of a reply line including the reply code and
3507 the <CRLF> is 512 octets. More information may be conveyed through
3508 multiple-line replies.
3510 4.5.3.1.6. Text Line
3512 The maximum total length of a text line including the <CRLF> is 1000
3513 octets (not counting the leading dot duplicated for transparency).
3514 This number may be increased by the use of SMTP Service Extensions.
3516 4.5.3.1.7. Message Content
3518 The maximum total length of a message content (including any message
3519 header section as well as the message body) MUST BE at least 64K
3520 octets. Since the introduction of Internet Standards for multimedia
3521 mail (RFC 2045 [21]), message lengths on the Internet have grown
3522 dramatically, and message size restrictions should be avoided if at
3523 all possible. SMTP server systems that must impose restrictions
3524 SHOULD implement the "SIZE" service extension of RFC 1870 [10], and
3525 SMTP client systems that will send large messages SHOULD utilize it
3526 when possible.
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3532 RFC 5321 SMTP October 2008
3535 4.5.3.1.8. Recipients Buffer
3537 The minimum total number of recipients that MUST be buffered is 100
3538 recipients. Rejection of messages (for excessive recipients) with
3539 fewer than 100 RCPT commands is a violation of this specification.
3540 The general principle that relaying SMTP server MUST NOT, and
3541 delivery SMTP servers SHOULD NOT, perform validation tests on message
3542 header fields suggests that messages SHOULD NOT be rejected based on
3543 the total number of recipients shown in header fields. A server that
3544 imposes a limit on the number of recipients MUST behave in an orderly
3545 fashion, such as rejecting additional addresses over its limit rather
3546 than silently discarding addresses previously accepted. A client
3547 that needs to deliver a message containing over 100 RCPT commands
3548 SHOULD be prepared to transmit in 100-recipient "chunks" if the
3549 server declines to accept more than 100 recipients in a single
3550 message.
3552 4.5.3.1.9. Treatment When Limits Exceeded
3554 Errors due to exceeding these limits may be reported by using the
3555 reply codes. Some examples of reply codes are:
3557 500 Line too long.
3559 or
3561 501 Path too long
3563 or
3565 452 Too many recipients (see below)
3567 or
3569 552 Too much mail data.
3571 4.5.3.1.10. Too Many Recipients Code
3573 RFC 821 [1] incorrectly listed the error where an SMTP server
3574 exhausts its implementation limit on the number of RCPT commands
3575 ("too many recipients") as having reply code 552. The correct reply
3576 code for this condition is 452. Clients SHOULD treat a 552 code in
3577 this case as a temporary, rather than permanent, failure so the logic
3578 below works.
3580 When a conforming SMTP server encounters this condition, it has at
3581 least 100 successful RCPT commands in its recipients buffer. If the
3582 server is able to accept the message, then at least these 100
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3588 RFC 5321 SMTP October 2008
3591 addresses will be removed from the SMTP client's queue. When the
3592 client attempts retransmission of those addresses that received 452
3593 responses, at least 100 of these will be able to fit in the SMTP
3594 server's recipients buffer. Each retransmission attempt that is able
3595 to deliver anything will be able to dispose of at least 100 of these
3596 recipients.
3598 If an SMTP server has an implementation limit on the number of RCPT
3599 commands and this limit is exhausted, it MUST use a response code of
3600 452 (but the client SHOULD also be prepared for a 552, as noted
3601 above). If the server has a configured site-policy limitation on the
3602 number of RCPT commands, it MAY instead use a 5yz response code. In
3603 particular, if the intent is to prohibit messages with more than a
3604 site-specified number of recipients, rather than merely limit the
3605 number of recipients in a given mail transaction, it would be
3606 reasonable to return a 503 response to any DATA command received
3607 subsequent to the 452 (or 552) code or to simply return the 503 after
3608 DATA without returning any previous negative response.
3610 4.5.3.2. Timeouts
3612 An SMTP client MUST provide a timeout mechanism. It MUST use per-
3613 command timeouts rather than somehow trying to time the entire mail
3614 transaction. Timeouts SHOULD be easily reconfigurable, preferably
3615 without recompiling the SMTP code. To implement this, a timer is set
3616 for each SMTP command and for each buffer of the data transfer. The
3617 latter means that the overall timeout is inherently proportional to
3618 the size of the message.
3620 Based on extensive experience with busy mail-relay hosts, the minimum
3621 per-command timeout values SHOULD be as follows:
3623 4.5.3.2.1. Initial 220 Message: 5 Minutes
3625 An SMTP client process needs to distinguish between a failed TCP
3626 connection and a delay in receiving the initial 220 greeting message.
3627 Many SMTP servers accept a TCP connection but delay delivery of the
3628 220 message until their system load permits more mail to be
3629 processed.
3631 4.5.3.2.2. MAIL Command: 5 Minutes
3633 4.5.3.2.3. RCPT Command: 5 Minutes
3635 A longer timeout is required if processing of mailing lists and
3636 aliases is not deferred until after the message was accepted.
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3644 RFC 5321 SMTP October 2008
3647 4.5.3.2.4. DATA Initiation: 2 Minutes
3649 This is while awaiting the "354 Start Input" reply to a DATA command.
3651 4.5.3.2.5. Data Block: 3 Minutes
3653 This is while awaiting the completion of each TCP SEND call
3654 transmitting a chunk of data.
3656 4.5.3.2.6. DATA Termination: 10 Minutes.
3658 This is while awaiting the "250 OK" reply. When the receiver gets
3659 the final period terminating the message data, it typically performs
3660 processing to deliver the message to a user mailbox. A spurious
3661 timeout at this point would be very wasteful and would typically
3662 result in delivery of multiple copies of the message, since it has
3663 been successfully sent and the server has accepted responsibility for
3664 delivery. See Section 6.1 for additional discussion.
3666 4.5.3.2.7. Server Timeout: 5 Minutes.
3668 An SMTP server SHOULD have a timeout of at least 5 minutes while it
3669 is awaiting the next command from the sender.
3671 4.5.4. Retry Strategies
3673 The common structure of a host SMTP implementation includes user
3674 mailboxes, one or more areas for queuing messages in transit, and one
3675 or more daemon processes for sending and receiving mail. The exact
3676 structure will vary depending on the needs of the users on the host
3677 and the number and size of mailing lists supported by the host. We
3678 describe several optimizations that have proved helpful, particularly
3679 for mailers supporting high traffic levels.
3681 Any queuing strategy MUST include timeouts on all activities on a
3682 per-command basis. A queuing strategy MUST NOT send error messages
3683 in response to error messages under any circumstances.
3685 4.5.4.1. Sending Strategy
3687 The general model for an SMTP client is one or more processes that
3688 periodically attempt to transmit outgoing mail. In a typical system,
3689 the program that composes a message has some method for requesting
3690 immediate attention for a new piece of outgoing mail, while mail that
3691 cannot be transmitted immediately MUST be queued and periodically
3692 retried by the sender. A mail queue entry will include not only the
3693 message itself but also the envelope information.
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3700 RFC 5321 SMTP October 2008
3703 The sender MUST delay retrying a particular destination after one
3704 attempt has failed. In general, the retry interval SHOULD be at
3705 least 30 minutes; however, more sophisticated and variable strategies
3706 will be beneficial when the SMTP client can determine the reason for
3707 non-delivery.
3709 Retries continue until the message is transmitted or the sender gives
3710 up; the give-up time generally needs to be at least 4-5 days. It MAY
3711 be appropriate to set a shorter maximum number of retries for non-
3712 delivery notifications and equivalent error messages than for
3713 standard messages. The parameters to the retry algorithm MUST be
3714 configurable.
3716 A client SHOULD keep a list of hosts it cannot reach and
3717 corresponding connection timeouts, rather than just retrying queued
3718 mail items.
3720 Experience suggests that failures are typically transient (the target
3721 system or its connection has crashed), favoring a policy of two
3722 connection attempts in the first hour the message is in the queue,
3723 and then backing off to one every two or three hours.
3725 The SMTP client can shorten the queuing delay in cooperation with the
3726 SMTP server. For example, if mail is received from a particular
3727 address, it is likely that mail queued for that host can now be sent.
3728 Application of this principle may, in many cases, eliminate the
3729 requirement for an explicit "send queues now" function such as ETRN,
3730 RFC 1985 [36].
3732 The strategy may be further modified as a result of multiple
3733 addresses per host (see below) to optimize delivery time versus
3734 resource usage.
3736 An SMTP client may have a large queue of messages for each
3737 unavailable destination host. If all of these messages were retried
3738 in every retry cycle, there would be excessive Internet overhead and
3739 the sending system would be blocked for a long period. Note that an
3740 SMTP client can generally determine that a delivery attempt has
3741 failed only after a timeout of several minutes, and even a one-minute
3742 timeout per connection will result in a very large delay if retries
3743 are repeated for dozens, or even hundreds, of queued messages to the
3744 same host.
3746 At the same time, SMTP clients SHOULD use great care in caching
3747 negative responses from servers. In an extreme case, if EHLO is
3748 issued multiple times during the same SMTP connection, different
3749 answers may be returned by the server. More significantly, 5yz
3750 responses to the MAIL command MUST NOT be cached.
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3756 RFC 5321 SMTP October 2008
3759 When a mail message is to be delivered to multiple recipients, and
3760 the SMTP server to which a copy of the message is to be sent is the
3761 same for multiple recipients, then only one copy of the message
3762 SHOULD be transmitted. That is, the SMTP client SHOULD use the
3763 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the
3764 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there
3765 are very many addresses, a limit on the number of RCPT commands per
3766 MAIL command MAY be imposed. This efficiency feature SHOULD be
3767 implemented.
3769 Similarly, to achieve timely delivery, the SMTP client MAY support
3770 multiple concurrent outgoing mail transactions. However, some limit
3771 may be appropriate to protect the host from devoting all its
3772 resources to mail.
3774 4.5.4.2. Receiving Strategy
3776 The SMTP server SHOULD attempt to keep a pending listen on the SMTP
3777 port (specified by IANA as port 25) at all times. This requires the
3778 support of multiple incoming TCP connections for SMTP. Some limit
3779 MAY be imposed, but servers that cannot handle more than one SMTP
3780 transaction at a time are not in conformance with the intent of this
3781 specification.
3783 As discussed above, when the SMTP server receives mail from a
3784 particular host address, it could activate its own SMTP queuing
3785 mechanisms to retry any mail pending for that host address.
3787 4.5.5. Messages with a Null Reverse-Path
3789 There are several types of notification messages that are required by
3790 existing and proposed Standards to be sent with a null reverse-path,
3791 namely non-delivery notifications as discussed in Section 3.7, other
3792 kinds of Delivery Status Notifications (DSNs, RFC 3461 [32]), and
3793 Message Disposition Notifications (MDNs, RFC 3798 [37]). All of
3794 these kinds of messages are notifications about a previous message,
3795 and they are sent to the reverse-path of the previous mail message.
3796 (If the delivery of such a notification message fails, that usually
3797 indicates a problem with the mail system of the host to which the
3798 notification message is addressed. For this reason, at some hosts
3799 the MTA is set up to forward such failed notification messages to
3800 someone who is able to fix problems with the mail system, e.g., via
3801 the postmaster alias.)
3803 All other types of messages (i.e., any message which is not required
3804 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent
3805 with a valid, non-null reverse-path.
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3812 RFC 5321 SMTP October 2008
3815 Implementers of automated email processors should be careful to make
3816 sure that the various kinds of messages with a null reverse-path are
3817 handled correctly. In particular, such systems SHOULD NOT reply to
3818 messages with a null reverse-path, and they SHOULD NOT add a non-null
3819 reverse-path, or change a null reverse-path to a non-null one, to
3820 such messages when forwarding.
3822 5. Address Resolution and Mail Handling
3824 5.1. Locating the Target Host
3826 Once an SMTP client lexically identifies a domain to which mail will
3827 be delivered for processing (as described in Sections 2.3.5 and 3.6),
3828 a DNS lookup MUST be performed to resolve the domain name (RFC 1035
3829 [2]). The names are expected to be fully-qualified domain names
3830 (FQDNs): mechanisms for inferring FQDNs from partial names or local
3831 aliases are outside of this specification. Due to a history of
3832 problems, SMTP servers used for initial submission of messages SHOULD
3833 NOT make such inferences (Message Submission Servers [18] have
3834 somewhat more flexibility) and intermediate (relay) SMTP servers MUST
3835 NOT make them.
3837 The lookup first attempts to locate an MX record associated with the
3838 name. If a CNAME record is found, the resulting name is processed as
3839 if it were the initial name. If a non-existent domain error is
3840 returned, this situation MUST be reported as an error. If a
3841 temporary error is returned, the message MUST be queued and retried
3842 later (see Section 4.5.4.1). If an empty list of MXs is returned,
3843 the address is treated as if it was associated with an implicit MX
3844 RR, with a preference of 0, pointing to that host. If MX records are
3845 present, but none of them are usable, or the implicit MX is unusable,
3846 this situation MUST be reported as an error.
3848 If one or more MX RRs are found for a given name, SMTP systems MUST
3849 NOT utilize any address RRs associated with that name unless they are
3850 located using the MX RRs; the "implicit MX" rule above applies only
3851 if there are no MX records present. If MX records are present, but
3852 none of them are usable, this situation MUST be reported as an error.
3854 When a domain name associated with an MX RR is looked up and the
3855 associated data field obtained, the data field of that response MUST
3856 contain a domain name. That domain name, when queried, MUST return
3857 at least one address record (e.g., A or AAAA RR) that gives the IP
3858 address of the SMTP server to which the message should be directed.
3859 Any other response, specifically including a value that will return a
3860 CNAME record when queried, lies outside the scope of this Standard.
3861 The prohibition on labels in the data that resolve to CNAMEs is
3862 discussed in more detail in RFC 2181, Section 10.3 [38].
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3868 RFC 5321 SMTP October 2008
3871 When the lookup succeeds, the mapping can result in a list of
3872 alternative delivery addresses rather than a single address, because
3873 of multiple MX records, multihoming, or both. To provide reliable
3874 mail transmission, the SMTP client MUST be able to try (and retry)
3875 each of the relevant addresses in this list in order, until a
3876 delivery attempt succeeds. However, there MAY also be a configurable
3877 limit on the number of alternate addresses that can be tried. In any
3878 case, the SMTP client SHOULD try at least two addresses.
3880 Two types of information are used to rank the host addresses:
3881 multiple MX records, and multihomed hosts.
3883 MX records contain a preference indication that MUST be used in
3884 sorting if more than one such record appears (see below). Lower
3885 numbers are more preferred than higher ones. If there are multiple
3886 destinations with the same preference and there is no clear reason to
3887 favor one (e.g., by recognition of an easily reached address), then
3888 the sender-SMTP MUST randomize them to spread the load across
3889 multiple mail exchangers for a specific organization.
3891 The destination host (perhaps taken from the preferred MX record) may
3892 be multihomed, in which case the domain name resolver will return a
3893 list of alternative IP addresses. It is the responsibility of the
3894 domain name resolver interface to have ordered this list by
3895 decreasing preference if necessary, and the SMTP sender MUST try them
3896 in the order presented.
3898 Although the capability to try multiple alternative addresses is
3899 required, specific installations may want to limit or disable the use
3900 of alternative addresses. The question of whether a sender should
3901 attempt retries using the different addresses of a multihomed host
3902 has been controversial. The main argument for using the multiple
3903 addresses is that it maximizes the probability of timely delivery,
3904 and indeed sometimes the probability of any delivery; the counter-
3905 argument is that it may result in unnecessary resource use. Note
3906 that resource use is also strongly determined by the sending strategy
3907 discussed in Section 4.5.4.1.
3909 If an SMTP server receives a message with a destination for which it
3910 is a designated Mail eXchanger, it MAY relay the message (potentially
3911 after having rewritten the MAIL FROM and/or RCPT TO addresses), make
3912 final delivery of the message, or hand it off using some mechanism
3913 outside the SMTP-provided transport environment. Of course, neither
3914 of the latter require that the list of MX records be examined
3915 further.
3917 If it determines that it should relay the message without rewriting
3918 the address, it MUST sort the MX records to determine candidates for
3922 Klensin Standards Track [Page 70]
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3924 RFC 5321 SMTP October 2008
3927 delivery. The records are first ordered by preference, with the
3928 lowest-numbered records being most preferred. The relay host MUST
3929 then inspect the list for any of the names or addresses by which it
3930 might be known in mail transactions. If a matching record is found,
3931 all records at that preference level and higher-numbered ones MUST be
3932 discarded from consideration. If there are no records left at that
3933 point, it is an error condition, and the message MUST be returned as
3934 undeliverable. If records do remain, they SHOULD be tried, best
3935 preference first, as described above.
3937 5.2. IPv6 and MX Records
3939 In the contemporary Internet, SMTP clients and servers may be hosted
3940 on IPv4 systems, IPv6 systems, or dual-stack systems that are
3941 compatible with either version of the Internet Protocol. The host
3942 domains to which MX records point may, consequently, contain "A RR"s
3943 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC
3944 3974 [39] discusses some operational experience in mixed
3945 environments, it was not comprehensive enough to justify
3946 standardization, and some of its recommendations appear to be
3947 inconsistent with this specification. The appropriate actions to be
3948 taken either will depend on local circumstances, such as performance
3949 of the relevant networks and any conversions that might be necessary,
3950 or will be obvious (e.g., an IPv6-only client need not attempt to
3951 look up A RRs or attempt to reach IPv4-only servers). Designers of
3952 SMTP implementations that might run in IPv6 or dual-stack
3953 environments should study the procedures above, especially the
3954 comments about multihomed hosts, and, preferably, provide mechanisms
3955 to facilitate operational tuning and mail interoperability between
3956 IPv4 and IPv6 systems while considering local circumstances.
3958 6. Problem Detection and Handling
3960 6.1. Reliable Delivery and Replies by Email
3962 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
3963 message in response to DATA), it is accepting responsibility for
3964 delivering or relaying the message. It must take this responsibility
3965 seriously. It MUST NOT lose the message for frivolous reasons, such
3966 as because the host later crashes or because of a predictable
3967 resource shortage. Some reasons that are not considered frivolous
3968 are discussed in the next subsection and in Section 7.8.
3970 If there is a delivery failure after acceptance of a message, the
3971 receiver-SMTP MUST formulate and mail a notification message. This
3972 notification MUST be sent using a null ("<>") reverse-path in the
3973 envelope. The recipient of this notification MUST be the address
3974 from the envelope return path (or the Return-Path: line). However,
3978 Klensin Standards Track [Page 71]
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3980 RFC 5321 SMTP October 2008
3983 if this address is null ("<>"), the receiver-SMTP MUST NOT send a
3984 notification. Obviously, nothing in this section can or should
3985 prohibit local decisions (i.e., as part of the same system
3986 environment as the receiver-SMTP) to log or otherwise transmit
3987 information about null address events locally if that is desired. If
3988 the address is an explicit source route, it MUST be stripped down to
3989 its final hop.
3991 For example, suppose that an error notification must be sent for a
3992 message that arrived with:
3994 MAIL FROM:<@a,@b:user@d>
3996 The notification message MUST be sent using:
3998 RCPT TO:<user@d>
4000 Some delivery failures after the message is accepted by SMTP will be
4001 unavoidable. For example, it may be impossible for the receiving
4002 SMTP server to validate all the delivery addresses in RCPT command(s)
4003 due to a "soft" domain system error, because the target is a mailing
4004 list (see earlier discussion of RCPT), or because the server is
4005 acting as a relay and has no immediate access to the delivering
4006 system.
4008 To avoid receiving duplicate messages as the result of timeouts, a
4009 receiver-SMTP MUST seek to minimize the time required to respond to
4010 the final <CRLF>.<CRLF> end of data indicator. See RFC 1047 [40] for
4011 a discussion of this problem.
4013 6.2. Unwanted, Unsolicited, and "Attack" Messages
4015 Utility and predictability of the Internet mail system requires that
4016 messages that can be delivered should be delivered, regardless of any
4017 syntax or other faults associated with those messages and regardless
4018 of their content. If they cannot be delivered, and cannot be
4019 rejected by the SMTP server during the SMTP transaction, they should
4020 be "bounced" (returned with non-delivery notification messages) as
4021 described above. In today's world, in which many SMTP server
4022 operators have discovered that the quantity of undesirable bulk email
4023 vastly exceeds the quantity of desired mail and in which accepting a
4024 message may trigger additional undesirable traffic by providing
4025 verification of the address, those principles may not be practical.
4027 As discussed in Section 7.8 and Section 7.9 below, dropping mail
4028 without notification of the sender is permitted in practice.
4029 However, it is extremely dangerous and violates a long tradition and
4030 community expectations that mail is either delivered or returned. If
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4036 RFC 5321 SMTP October 2008
4039 silent message-dropping is misused, it could easily undermine
4040 confidence in the reliability of the Internet's mail systems. So
4041 silent dropping of messages should be considered only in those cases
4042 where there is very high confidence that the messages are seriously
4043 fraudulent or otherwise inappropriate.
4045 To stretch the principle of delivery if possible even further, it may
4046 be a rational policy to not deliver mail that has an invalid return
4047 address, although the history of the network is that users are
4048 typically better served by delivering any message that can be
4049 delivered. Reliably determining that a return address is invalid can
4050 be a difficult and time-consuming process, especially if the putative
4051 sending system is not directly accessible or does not fully and
4052 accurately support VRFY and, even if a "drop messages with invalid
4053 return addresses" policy is adopted, it SHOULD be applied only when
4054 there is near-certainty that the return addresses are, in fact,
4055 invalid.
4057 Conversely, if a message is rejected because it is found to contain
4058 hostile content (a decision that is outside the scope of an SMTP
4059 server as defined in this document), rejection ("bounce") messages
4060 SHOULD NOT be sent unless the receiving site is confident that those
4061 messages will be usefully delivered. The preference and default in
4062 these cases is to avoid sending non-delivery messages when the
4063 incoming message is determined to contain hostile content.
4065 6.3. Loop Detection
4067 Simple counting of the number of "Received:" header fields in a
4068 message has proven to be an effective, although rarely optimal,
4069 method of detecting loops in mail systems. SMTP servers using this
4070 technique SHOULD use a large rejection threshold, normally at least
4071 100 Received entries. Whatever mechanisms are used, servers MUST
4072 contain provisions for detecting and stopping trivial loops.
4074 6.4. Compensating for Irregularities
4076 Unfortunately, variations, creative interpretations, and outright
4077 violations of Internet mail protocols do occur; some would suggest
4078 that they occur quite frequently. The debate as to whether a well-
4079 behaved SMTP receiver or relay should reject a malformed message,
4080 attempt to pass it on unchanged, or attempt to repair it to increase
4081 the odds of successful delivery (or subsequent reply) began almost
4082 with the dawn of structured network mail and shows no signs of
4083 abating. Advocates of rejection claim that attempted repairs are
4084 rarely completely adequate and that rejection of bad messages is the
4085 only way to get the offending software repaired. Advocates of
4086 "repair" or "deliver no matter what" argue that users prefer that
4090 Klensin Standards Track [Page 73]
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4092 RFC 5321 SMTP October 2008
4095 mail go through it if at all possible and that there are significant
4096 market pressures in that direction. In practice, these market
4097 pressures may be more important to particular vendors than strict
4098 conformance to the standards, regardless of the preference of the
4099 actual developers.
4101 The problems associated with ill-formed messages were exacerbated by
4102 the introduction of the split-UA mail reading protocols (Post Office
4103 Protocol (POP) version 2 [15], Post Office Protocol (POP) version 3
4104 [16], IMAP version 2 [41], and PCMAIL [42]). These protocols
4105 encouraged the use of SMTP as a posting (message submission)
4106 protocol, and SMTP servers as relay systems for these client hosts
4107 (which are often only intermittently connected to the Internet).
4108 Historically, many of those client machines lacked some of the
4109 mechanisms and information assumed by SMTP (and indeed, by the mail
4110 format protocol, RFC 822 [28]). Some could not keep adequate track
4111 of time; others had no concept of time zones; still others could not
4112 identify their own names or addresses; and, of course, none could
4113 satisfy the assumptions that underlay RFC 822's conception of
4114 authenticated addresses.
4116 In response to these weak SMTP clients, many SMTP systems now
4117 complete messages that are delivered to them in incomplete or
4118 incorrect form. This strategy is generally considered appropriate
4119 when the server can identify or authenticate the client, and there
4120 are prior agreements between them. By contrast, there is at best
4121 great concern about fixes applied by a relay or delivery SMTP server
4122 that has little or no knowledge of the user or client machine. Many
4123 of these issues are addressed by using a separate protocol, such as
4124 that defined in RFC 4409 [18], for message submission, rather than
4125 using originating SMTP servers for that purpose.
4127 The following changes to a message being processed MAY be applied
4128 when necessary by an originating SMTP server, or one used as the
4129 target of SMTP as an initial posting (message submission) protocol:
4131 o Addition of a message-id field when none appears
4133 o Addition of a date, time, or time zone when none appears
4135 o Correction of addresses to proper FQDN format
4137 The less information the server has about the client, the less likely
4138 these changes are to be correct and the more caution and conservatism
4139 should be applied when considering whether or not to perform fixes
4140 and how. These changes MUST NOT be applied by an SMTP server that
4141 provides an intermediate relay function.
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4148 RFC 5321 SMTP October 2008
4151 In all cases, properly operating clients supplying correct
4152 information are preferred to corrections by the SMTP server. In all
4153 cases, documentation SHOULD be provided in trace header fields and/or
4154 header field comments for actions performed by the servers.
4156 7. Security Considerations
4158 7.1. Mail Security and Spoofing
4160 SMTP mail is inherently insecure in that it is feasible for even
4161 fairly casual users to negotiate directly with receiving and relaying
4162 SMTP servers and create messages that will trick a naive recipient
4163 into believing that they came from somewhere else. Constructing such
4164 a message so that the "spoofed" behavior cannot be detected by an
4165 expert is somewhat more difficult, but not sufficiently so as to be a
4166 deterrent to someone who is determined and knowledgeable.
4167 Consequently, as knowledge of Internet mail increases, so does the
4168 knowledge that SMTP mail inherently cannot be authenticated, or
4169 integrity checks provided, at the transport level. Real mail
4170 security lies only in end-to-end methods involving the message
4171 bodies, such as those that use digital signatures (see RFC 1847 [43]
4172 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [44] or Secure/
4173 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [45]).
4175 Various protocol extensions and configuration options that provide
4176 authentication at the transport level (e.g., from an SMTP client to
4177 an SMTP server) improve somewhat on the traditional situation
4178 described above. However, in general, they only authenticate one
4179 server to another rather than a chain of relays and servers, much
4180 less authenticating users or user machines. Consequently, unless
4181 they are accompanied by careful handoffs of responsibility in a
4182 carefully designed trust environment, they remain inherently weaker
4183 than end-to-end mechanisms that use digitally signed messages rather
4184 than depending on the integrity of the transport system.
4186 Efforts to make it more difficult for users to set envelope return
4187 path and header "From" fields to point to valid addresses other than
4188 their own are largely misguided: they frustrate legitimate
4189 applications in which mail is sent by one user on behalf of another,
4190 in which error (or normal) replies should be directed to a special
4191 address, or in which a single message is sent to multiple recipients
4192 on different hosts. (Systems that provide convenient ways for users
4193 to alter these header fields on a per-message basis should attempt to
4194 establish a primary and permanent mailbox address for the user so
4195 that Sender header fields within the message data can be generated
4196 sensibly.)
4202 Klensin Standards Track [Page 75]
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4204 RFC 5321 SMTP October 2008
4207 This specification does not further address the authentication issues
4208 associated with SMTP other than to advocate that useful functionality
4209 not be disabled in the hope of providing some small margin of
4210 protection against a user who is trying to fake mail.
4212 7.2. "Blind" Copies
4214 Addresses that do not appear in the message header section may appear
4215 in the RCPT commands to an SMTP server for a number of reasons. The
4216 two most common involve the use of a mailing address as a "list
4217 exploder" (a single address that resolves into multiple addresses)
4218 and the appearance of "blind copies". Especially when more than one
4219 RCPT command is present, and in order to avoid defeating some of the
4220 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
4221 the full set of RCPT command arguments into the header section,
4222 either as part of trace header fields or as informational or private-
4223 extension header fields. Since this rule is often violated in
4224 practice, and cannot be enforced, sending SMTP systems that are aware
4225 of "bcc" use MAY find it helpful to send each blind copy as a
4226 separate message transaction containing only a single RCPT command.
4228 There is no inherent relationship between either "reverse" (from
4229 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
4230 transaction ("envelope") and the addresses in the header section.
4231 Receiving systems SHOULD NOT attempt to deduce such relationships and
4232 use them to alter the header section of the message for delivery.
4233 The popular "Apparently-to" header field is a violation of this
4234 principle as well as a common source of unintended information
4235 disclosure and SHOULD NOT be used.
4237 7.3. VRFY, EXPN, and Security
4239 As discussed in Section 3.5, individual sites may want to disable
4240 either or both of VRFY or EXPN for security reasons (see below). As
4241 a corollary to the above, implementations that permit this MUST NOT
4242 appear to have verified addresses that are not, in fact, verified.
4243 If a site disables these commands for security reasons, the SMTP
4244 server MUST return a 252 response, rather than a code that could be
4245 confused with successful or unsuccessful verification.
4247 Returning a 250 reply code with the address listed in the VRFY
4248 command after having checked it only for syntax violates this rule.
4249 Of course, an implementation that "supports" VRFY by always returning
4250 550 whether or not the address is valid is equally not in
4251 conformance.
4253 On the public Internet, the contents of mailing lists have become
4254 popular as an address information source for so-called "spammers."
4258 Klensin Standards Track [Page 76]
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4260 RFC 5321 SMTP October 2008
4263 The use of EXPN to "harvest" addresses has increased as list
4264 administrators have installed protections against inappropriate uses
4265 of the lists themselves. However, VRFY and EXPN are still useful for
4266 authenticated users and within an administrative domain. For
4267 example, VRFY and EXPN are useful for performing internal audits of
4268 how email gets routed to check and to make sure no one is
4269 automatically forwarding sensitive mail outside the organization.
4270 Sites implementing SMTP authentication may choose to make VRFY and
4271 EXPN available only to authenticated requestors. Implementations
4272 SHOULD still provide support for EXPN, but sites SHOULD carefully
4273 evaluate the tradeoffs.
4275 Whether disabling VRFY provides any real marginal security depends on
4276 a series of other conditions. In many cases, RCPT commands can be
4277 used to obtain the same information about address validity. On the
4278 other hand, especially in situations where determination of address
4279 validity for RCPT commands is deferred until after the DATA command
4280 is received, RCPT may return no information at all, while VRFY is
4281 expected to make a serious attempt to determine validity before
4282 generating a response code (see discussion above).
4284 7.4. Mail Rerouting Based on the 251 and 551 Response Codes
4286 Before a client uses the 251 or 551 reply codes from a RCPT command
4287 to automatically update its future behavior (e.g., updating the
4288 user's address book), it should be certain of the server's
4289 authenticity. If it does not, it may be subject to a man in the
4290 middle attack.
4292 7.5. Information Disclosure in Announcements
4294 There has been an ongoing debate about the tradeoffs between the
4295 debugging advantages of announcing server type and version (and,
4296 sometimes, even server domain name) in the greeting response or in
4297 response to the HELP command and the disadvantages of exposing
4298 information that might be useful in a potential hostile attack. The
4299 utility of the debugging information is beyond doubt. Those who
4300 argue for making it available point out that it is far better to
4301 actually secure an SMTP server rather than hope that trying to
4302 conceal known vulnerabilities by hiding the server's precise identity
4303 will provide more protection. Sites are encouraged to evaluate the
4304 tradeoff with that issue in mind; implementations SHOULD minimally
4305 provide for making type and version information available in some way
4306 to other network hosts.
4314 Klensin Standards Track [Page 77]
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4316 RFC 5321 SMTP October 2008
4319 7.6. Information Disclosure in Trace Fields
4321 In some circumstances, such as when mail originates from within a LAN
4322 whose hosts are not directly on the public Internet, trace
4323 ("Received") header fields produced in conformance with this
4324 specification may disclose host names and similar information that
4325 would not normally be available. This ordinarily does not pose a
4326 problem, but sites with special concerns about name disclosure should
4327 be aware of it. Also, the optional FOR clause should be supplied
4328 with caution or not at all when multiple recipients are involved lest
4329 it inadvertently disclose the identities of "blind copy" recipients
4330 to others.
4332 7.7. Information Disclosure in Message Forwarding
4334 As discussed in Section 3.4, use of the 251 or 551 reply codes to
4335 identify the replacement address associated with a mailbox may
4336 inadvertently disclose sensitive information. Sites that are
4337 concerned about those issues should ensure that they select and
4338 configure servers appropriately.
4340 7.8. Resistance to Attacks
4342 In recent years, there has been an increase of attacks on SMTP
4343 servers, either in conjunction with attempts to discover addresses
4344 for sending unsolicited messages or simply to make the servers
4345 inaccessible to others (i.e., as an application-level denial of
4346 service attack). While the means of doing so are beyond the scope of
4347 this Standard, rational operational behavior requires that servers be
4348 permitted to detect such attacks and take action to defend
4349 themselves. For example, if a server determines that a large number
4350 of RCPT TO commands are being sent, most or all with invalid
4351 addresses, as part of such an attack, it would be reasonable for the
4352 server to close the connection after generating an appropriate number
4353 of 5yz (normally 550) replies.
4355 7.9. Scope of Operation of SMTP Servers
4357 It is a well-established principle that an SMTP server may refuse to
4358 accept mail for any operational or technical reason that makes sense
4359 to the site providing the server. However, cooperation among sites
4360 and installations makes the Internet possible. If sites take
4361 excessive advantage of the right to reject traffic, the ubiquity of
4362 email availability (one of the strengths of the Internet) will be
4363 threatened; considerable care should be taken and balance maintained
4364 if a site decides to be selective about the traffic it will accept
4365 and process.
4370 Klensin Standards Track [Page 78]
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4372 RFC 5321 SMTP October 2008
4375 In recent years, use of the relay function through arbitrary sites
4376 has been used as part of hostile efforts to hide the actual origins
4377 of mail. Some sites have decided to limit the use of the relay
4378 function to known or identifiable sources, and implementations SHOULD
4379 provide the capability to perform this type of filtering. When mail
4380 is rejected for these or other policy reasons, a 550 code SHOULD be
4381 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.
4383 8. IANA Considerations
4385 IANA maintains three registries in support of this specification, all
4386 of which were created for RFC 2821 or earlier. This document expands
4387 the third one as specified below. The registry references listed are
4388 as of the time of publication; IANA does not guarantee the locations
4389 associated with the URLs. The registries are as follows:
4391 o The first, "Simple Mail Transfer Protocol (SMTP) Service
4392 Extensions" [46], consists of SMTP service extensions with the
4393 associated keywords, and, as needed, parameters and verbs. As
4394 specified in Section 2.2.2, no entry may be made in this registry
4395 that starts in an "X". Entries may be made only for service
4396 extensions (and associated keywords, parameters, or verbs) that
4397 are defined in Standards-Track or Experimental RFCs specifically
4398 approved by the IESG for this purpose.
4400 o The second registry, "Address Literal Tags" [47], consists of
4401 "tags" that identify forms of domain literals other than those for
4402 IPv4 addresses (specified in RFC 821 and in this document). The
4403 initial entry in that registry is for IPv6 addresses (specified in
4404 this document). Additional literal types require standardization
4405 before being used; none are anticipated at this time.
4407 o The third, "Mail Transmission Types" [46], established by RFC 821
4408 and renewed by this specification, is a registry of link and
4409 protocol identifiers to be used with the "via" and "with"
4410 subclauses of the time stamp ("Received:" header field) described
4411 in Section 4.4. Link and protocol identifiers in addition to
4412 those specified in this document may be registered only by
4413 standardization or by way of an RFC-documented, IESG-approved,
4414 Experimental protocol extension. This name space is for
4415 identification and not limited in size: the IESG is encouraged to
4416 approve on the basis of clear documentation and a distinct method
4417 rather than preferences about the properties of the method itself.
4419 An additional subsection has been added to the "VIA link types"
4420 and "WITH protocol types" subsections of this registry to contain
4421 registrations of "Additional-registered-clauses" as described
4422 above. The registry will contain clause names, a description, a
4426 Klensin Standards Track [Page 79]
4427 \f
4428 RFC 5321 SMTP October 2008
4431 summary of the syntax of the associated String, and a reference.
4432 As new clauses are defined, they may, in principle, specify
4433 creation of their own registries if the Strings consist of
4434 reserved terms or keywords rather than less restricted strings.
4435 As with link and protocol identifiers, additional clauses may be
4436 registered only by standardization or by way of an RFC-documented,
4437 IESG-approved, Experimental protocol extension. The additional
4438 clause name space is for identification and is not limited in
4439 size: the IESG is encouraged to approve on the basis of clear
4440 documentation, actual use or strong signs that the clause will be
4441 used, and a distinct requirement rather than preferences about the
4442 properties of the clause itself.
4444 In addition, if additional trace header fields (i.e., in addition to
4445 Return-path and Received) are ever created, those trace fields MUST
4446 be added to the IANA registry established by BCP 90 (RFC 3864) [11]
4447 for use with RFC 5322 [4].
4449 9. Acknowledgments
4451 Many people contributed to the development of RFC 2821. That
4452 document should be consulted for those acknowledgments. For the
4453 present document, the editor and the community owe thanks to Dawn
4454 Mann and Tony Hansen who assisted in the very painful process of
4455 editing and converting the internal format of the document from one
4456 system to another.
4458 Neither this document nor RFC 2821 would have been possible without
4459 the many contribution and insights of the late Jon Postel. Those
4460 contributions of course include the original specification of SMTP in
4461 RFC 821. A considerable quantity of text from RFC 821 still appears
4462 in this document as do several of Jon's original examples that have
4463 been updated only as needed to reflect other changes in the
4464 specification.
4466 Many people made comments or suggestions on the mailing list or in
4467 notes to the author. Important corrections or clarifications were
4468 suggested by several people, including Matti Aarnio, Glenn Anderson,
4469 Derek J. Balling, Alex van den Bogaerdt, Stephane Bortzmeyer, Vint
4470 Cerf, Jutta Degener, Steve Dorner, Lisa Dusseault, Frank Ellerman,
4471 Ned Freed, Randy Gellens, Sabahattin Gucukoglu, Philip Guenther, Arnt
4472 Gulbrandsen, Eric Hall, Richard O. Hammer, Tony Hansen, Peter J.
4473 Holzer, Kari Hurtta, Bryon Roche Kain, Valdis Kletnieks, Mathias
4474 Koerber, John Leslie, Bruce Lilly, Jeff Macdonald, Mark E. Mallett,
4475 Mark Martinec, S. Moonesamy, Lyndon Nerenberg, Chris Newman, Douglas
4476 Otis, Pete Resnick, Robert A. Rosenberg, Vince Sabio, Hector Santos,
4477 David F. Skoll, Paul Smith, and Brett Watson.
4482 Klensin Standards Track [Page 80]
4483 \f
4484 RFC 5321 SMTP October 2008
4487 The efforts of the Area Directors -- Lisa Dusseault, Ted Hardie, and
4488 Chris Newman -- to get this effort restarted and keep it moving, and
4489 of an ad hoc committee with the same purpose, are gratefully
4490 acknowledged. The members of that committee were (in alphabetical
4491 order) Dave Crocker, Cyrus Daboo, Tony Finch, Ned Freed, Randall
4492 Gellens, Tony Hansen, the author, and Alexey Melnikov. Tony Hansen
4493 also acted as ad hoc chair on the mailing list reviewing this
4494 document; without his efforts, sense of balance and fairness, and
4495 patience, it clearly would not have been possible.
4497 10. References
4499 10.1. Normative References
4501 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
4502 August 1982.
4504 [2] Mockapetris, P., "Domain names - implementation and
4505 specification", STD 13, RFC 1035, November 1987.
4507 [3] Braden, R., "Requirements for Internet Hosts - Application and
4508 Support", STD 3, RFC 1123, October 1989.
4510 [4] Resnick, P., "Internet Message Format", RFC 5322, October 2008.
4512 [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
4513 Levels", BCP 14, RFC 2119, March 1997.
4515 [6] American National Standards Institute (formerly United States
4516 of America Standards Institute), "USA Code for Information
4517 Interchange", ANSI X3.4-1968, 1968.
4519 ANSI X3.4-1968 has been replaced by newer versions with slight
4520 modifications, but the 1968 version remains definitive for the
4521 Internet.
4523 [7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
4524 Specifications: ABNF", STD 68, RFC 5234, January 2008.
4526 [8] Hinden, R. and S. Deering, "IP Version 6 Addressing
4527 Architecture", RFC 4291, February 2006.
4529 [9] Newman, C., "ESMTP and LMTP Transmission Types Registration",
4530 RFC 3848, July 2004.
4532 [10] Klensin, J., Freed, N., and K. Moore, "SMTP Service Extension
4533 for Message Size Declaration", STD 10, RFC 1870, November 1995.
4538 Klensin Standards Track [Page 81]
4539 \f
4540 RFC 5321 SMTP October 2008
4543 [11] Klyne, G., Nottingham, M., and J. Mogul, "Registration
4544 Procedures for Message Header Fields", BCP 90, RFC 3864,
4545 September 2004.
4547 10.2. Informative References
4549 [12] Partridge, C., "Mail routing and the domain system", RFC 974,
4550 January 1986.
4552 [13] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
4553 Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
4554 November 1995.
4556 [14] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
4557 April 2001.
4559 [15] Butler, M., Postel, J., Chase, D., Goldberger, J., and J.
4560 Reynolds, "Post Office Protocol: Version 2", RFC 937,
4561 February 1985.
4563 [16] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
4564 STD 53, RFC 1939, May 1996.
4566 [17] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4567 4rev1", RFC 3501, March 2003.
4569 [18] Gellens, R. and J. Klensin, "Message Submission for Mail",
4570 RFC 4409, April 2006.
4572 [19] Freed, N., "SMTP Service Extension for Command Pipelining",
4573 STD 60, RFC 2920, September 2000.
4575 [20] Vaudreuil, G., "SMTP Service Extensions for Transmission of
4576 Large and Binary MIME Messages", RFC 3030, December 2000.
4578 [21] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
4579 Extensions (MIME) Part One: Format of Internet Message Bodies",
4580 RFC 2045, November 1996.
4582 [22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
4583 Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
4584 RFC 1652, July 1994.
4586 [23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
4587 Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
4588 November 1996.
4594 Klensin Standards Track [Page 82]
4595 \f
4596 RFC 5321 SMTP October 2008
4599 [24] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
4600 Extensions: Character Sets, Languages, and Continuations",
4601 RFC 2231, November 1997.
4603 [25] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
4604 January 2003.
4606 [26] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail
4607 System Status Codes", BCP 138, RFC 5248, June 2008.
4609 [27] Freed, N., "Behavior of and Requirements for Internet
4610 Firewalls", RFC 2979, October 2000.
4612 [28] Crocker, D., "Standard for the format of ARPA Internet text
4613 messages", STD 11, RFC 822, August 1982.
4615 [29] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for
4616 Authorizing Use of Domains in E-Mail, Version 1", RFC 4408,
4617 April 2006.
4619 [30] Fenton, J., "Analysis of Threats Motivating DomainKeys
4620 Identified Mail (DKIM)", RFC 4686, September 2006.
4622 [31] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and
4623 M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",
4624 RFC 4871, May 2007.
4626 [32] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
4627 Extension for Delivery Status Notifications (DSNs)", RFC 3461,
4628 January 2003.
4630 [33] Moore, K. and G. Vaudreuil, "An Extensible Message Format for
4631 Delivery Status Notifications", RFC 3464, January 2003.
4633 [34] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
4634 RFC 959, October 1985.
4636 [35] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
4637 between X.400 and RFC 822/MIME", RFC 2156, January 1998.
4639 [36] De Winter, J., "SMTP Service Extension for Remote Message Queue
4640 Starting", RFC 1985, August 1996.
4642 [37] Hansen, T. and G. Vaudreuil, "Message Disposition
4643 Notification", RFC 3798, May 2004.
4645 [38] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
4646 RFC 2181, July 1997.
4650 Klensin Standards Track [Page 83]
4651 \f
4652 RFC 5321 SMTP October 2008
4655 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience in
4656 Mixed IPv4/v6 Environments", RFC 3974, January 2005.
4658 [40] Partridge, C., "Duplicate messages and SMTP", RFC 1047,
4659 February 1988.
4661 [41] Crispin, M., "Interactive Mail Access Protocol: Version 2",
4662 RFC 1176, August 1990.
4664 [42] Lambert, M., "PCMAIL: A distributed mail system for personal
4665 computers", RFC 1056, June 1988.
4667 [43] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
4668 Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
4669 RFC 1847, October 1995.
4671 [44] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
4672 Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
4674 [45] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
4675 (S/MIME) Version 3.1 Message Specification", RFC 3851,
4676 July 2004.
4678 [46] Internet Assigned Number Authority (IANA), "IANA Mail
4679 Parameters", 2007,
4680 <http://www.iana.org/assignments/mail-parameters>.
4682 [47] Internet Assigned Number Authority (IANA), "Address Literal
4683 Tags", 2007,
4684 <http://www.iana.org/assignments/address-literal-tags>.
4706 Klensin Standards Track [Page 84]
4707 \f
4708 RFC 5321 SMTP October 2008
4711 Appendix A. TCP Transport Service
4713 The TCP connection supports the transmission of 8-bit bytes. The
4714 SMTP data is 7-bit ASCII characters. Each character is transmitted
4715 as an 8-bit byte with the high-order bit cleared to zero. Service
4716 extensions may modify this rule to permit transmission of full 8-bit
4717 data bytes as part of the message body, or, if specifically designed
4718 to do so, in SMTP commands or responses.
4720 Appendix B. Generating SMTP Commands from RFC 822 Header Fields
4722 Some systems use an RFC 822 header section (only) in a mail
4723 submission protocol, or otherwise generate SMTP commands from RFC 822
4724 header fields when such a message is handed to an MTA from a UA.
4725 While the MTA-UA protocol is a private matter, not covered by any
4726 Internet Standard, there are problems with this approach. For
4727 example, there have been repeated problems with proper handling of
4728 "bcc" copies and redistribution lists when information that
4729 conceptually belongs to the mail envelope is not separated early in
4730 processing from header field information (and kept separate).
4732 It is recommended that the UA provide its initial ("submission
4733 client") MTA with an envelope separate from the message itself.
4734 However, if the envelope is not supplied, SMTP commands SHOULD be
4735 generated as follows:
4737 1. Each recipient address from a TO, CC, or BCC header field SHOULD
4738 be copied to a RCPT command (generating multiple message copies
4739 if that is required for queuing or delivery). This includes any
4740 addresses listed in a RFC 822 "group". Any BCC header fields
4741 SHOULD then be removed from the header section. Once this
4742 process is completed, the remaining header fields SHOULD be
4743 checked to verify that at least one TO, CC, or BCC header field
4744 remains. If none do, then a BCC header field with no additional
4745 information SHOULD be inserted as specified in [4].
4747 2. The return address in the MAIL command SHOULD, if possible, be
4748 derived from the system's identity for the submitting (local)
4749 user, and the "From:" header field otherwise. If there is a
4750 system identity available, it SHOULD also be copied to the Sender
4751 header field if it is different from the address in the From
4752 header field. (Any Sender header field that was already there
4753 SHOULD be removed.) Systems may provide a way for submitters to
4754 override the envelope return address, but may want to restrict
4755 its use to privileged users. This will not prevent mail forgery,
4756 but may lessen its incidence; see Section 7.1.
4762 Klensin Standards Track [Page 85]
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4764 RFC 5321 SMTP October 2008
4767 When an MTA is being used in this way, it bears responsibility for
4768 ensuring that the message being transmitted is valid. The mechanisms
4769 for checking that validity, and for handling (or returning) messages
4770 that are not valid at the time of arrival, are part of the MUA-MTA
4771 interface and not covered by this specification.
4773 A submission protocol based on Standard RFC 822 information alone
4774 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
4775 system into an SMTP environment. Additional information to construct
4776 an envelope must come from some source in the other environment,
4777 whether supplemental header fields or the foreign system's envelope.
4779 Attempts to gateway messages using only their header "To" and "Cc"
4780 fields have repeatedly caused mail loops and other behavior adverse
4781 to the proper functioning of the Internet mail environment. These
4782 problems have been especially common when the message originates from
4783 an Internet mailing list and is distributed into the foreign
4784 environment using envelope information. When these messages are then
4785 processed by a header-section-only remailer, loops back to the
4786 Internet environment (and the mailing list) are almost inevitable.
4788 Appendix C. Source Routes
4790 Historically, the <reverse-path> was a reverse source routing list of
4791 hosts and a source mailbox. The first host in the <reverse-path> was
4792 historically the host sending the MAIL command; today, source routes
4793 SHOULD NOT appear in the reverse-path. Similarly, the <forward-path>
4794 may be a source routing lists of hosts and a destination mailbox.
4795 However, in general, the <forward-path> SHOULD contain only a mailbox
4796 and domain name, relying on the domain name system to supply routing
4797 information if required. The use of source routes is deprecated (see
4798 Appendix F.2); while servers MUST be prepared to receive and handle
4799 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT
4800 transmit them and this section is included in the current
4801 specification only to provide context. It has been modified somewhat
4802 from the material in RFC 821 to prevent server actions that might
4803 confuse clients or subsequent servers that do not expect a full
4804 source route implementation.
4806 For relay purposes, the forward-path may be a source route of the
4807 form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-
4808 qualified domain names. This form is used to emphasize the
4809 distinction between an address and a route. The mailbox (here, JOE@
4810 THREE) is an absolute address, and the route is information about how
4811 to get there. The two concepts should not be confused.
4813 If source routes are used, RFC 821 and the text below should be
4814 consulted for the mechanisms for constructing and updating the
4818 Klensin Standards Track [Page 86]
4819 \f
4820 RFC 5321 SMTP October 2008
4823 forward-path. A server that is reached by means of a source route
4824 (e.g., its domain name appears first in the list in the forward-path)
4825 MUST remove its domain name from any forward-paths in which that
4826 domain name appears before forwarding the message and MAY remove all
4827 other source routing information. The reverse-path SHOULD NOT be
4828 updated by servers conforming to this specification.
4830 Notice that the forward-path and reverse-path appear in the SMTP
4831 commands and replies, but not necessarily in the message. That is,
4832 there is no need for these paths and especially this syntax to appear
4833 in the "To:" , "From:", "CC:", etc. fields of the message header
4834 section. Conversely, SMTP servers MUST NOT derive final message
4835 routing information from message header fields.
4837 When the list of hosts is present despite the recommendations above,
4838 it is a "reverse" source route and indicates that the mail was
4839 relayed through each host on the list (the first host in the list was
4840 the most recent relay). This list is used as a source route to
4841 return non-delivery notices to the sender. If, contrary to the
4842 recommendations here, a relay host adds itself to the beginning of
4843 the list, it MUST use its name as known in the transport environment
4844 to which it is relaying the mail rather than that of the transport
4845 environment from which the mail came (if they are different). Note
4846 that a situation could easily arise in which some relay hosts add
4847 their names to the reverse source route and others do not, generating
4848 discontinuities in the routing list. This is another reason why
4849 servers needing to return a message SHOULD ignore the source route
4850 entirely and simply use the domain as specified in the Mailbox.
4852 Appendix D. Scenarios
4854 This section presents complete scenarios of several types of SMTP
4855 sessions. In the examples, "C:" indicates what is said by the SMTP
4856 client, and "S:" indicates what is said by the SMTP server.
4874 Klensin Standards Track [Page 87]
4875 \f
4876 RFC 5321 SMTP October 2008
4879 D.1. A Typical SMTP Transaction Scenario
4881 This SMTP example shows mail sent by Smith at host bar.com, and to
4882 Jones, Green, and Brown at host foo.com. Here we assume that host
4883 bar.com contacts host foo.com directly. The mail is accepted for
4884 Jones and Brown. Green does not have a mailbox at host foo.com.
4886 S: 220 foo.com Simple Mail Transfer Service Ready
4887 C: EHLO bar.com
4888 S: 250-foo.com greets bar.com
4889 S: 250-8BITMIME
4890 S: 250-SIZE
4891 S: 250-DSN
4892 S: 250 HELP
4893 C: MAIL FROM:<Smith@bar.com>
4894 S: 250 OK
4895 C: RCPT TO:<Jones@foo.com>
4896 S: 250 OK
4897 C: RCPT TO:<Green@foo.com>
4898 S: 550 No such user here
4899 C: RCPT TO:<Brown@foo.com>
4900 S: 250 OK
4901 C: DATA
4902 S: 354 Start mail input; end with <CRLF>.<CRLF>
4903 C: Blah blah blah...
4904 C: ...etc. etc. etc.
4905 C: .
4906 S: 250 OK
4907 C: QUIT
4908 S: 221 foo.com Service closing transmission channel
4930 Klensin Standards Track [Page 88]
4931 \f
4932 RFC 5321 SMTP October 2008
4935 D.2. Aborted SMTP Transaction Scenario
4937 S: 220 foo.com Simple Mail Transfer Service Ready
4938 C: EHLO bar.com
4939 S: 250-foo.com greets bar.com
4940 S: 250-8BITMIME
4941 S: 250-SIZE
4942 S: 250-DSN
4943 S: 250 HELP
4944 C: MAIL FROM:<Smith@bar.com>
4945 S: 250 OK
4946 C: RCPT TO:<Jones@foo.com>
4947 S: 250 OK
4948 C: RCPT TO:<Green@foo.com>
4949 S: 550 No such user here
4950 C: RSET
4951 S: 250 OK
4952 C: QUIT
4953 S: 221 foo.com Service closing transmission channel
4986 Klensin Standards Track [Page 89]
4987 \f
4988 RFC 5321 SMTP October 2008
4991 D.3. Relayed Mail Scenario
4993 Step 1 -- Source Host to Relay Host
4995 The source host performs a DNS lookup on XYZ.COM (the destination
4996 address) and finds DNS MX records specifying xyz.com as the best
4997 preference and foo.com as a lower preference. It attempts to open a
4998 connection to xyz.com and fails. It then opens a connection to
4999 foo.com, with the following dialogue:
5001 S: 220 foo.com Simple Mail Transfer Service Ready
5002 C: EHLO bar.com
5003 S: 250-foo.com greets bar.com
5004 S: 250-8BITMIME
5005 S: 250-SIZE
5006 S: 250-DSN
5007 S: 250 HELP
5008 C: MAIL FROM:<JQP@bar.com>
5009 S: 250 OK
5010 C: RCPT TO:<Jones@XYZ.COM>
5011 S: 250 OK
5012 C: DATA
5013 S: 354 Start mail input; end with <CRLF>.<CRLF>
5014 C: Date: Thu, 21 May 1998 05:33:29 -0700
5015 C: From: John Q. Public <JQP@bar.com>
5016 C: Subject: The Next Meeting of the Board
5017 C: To: Jones@xyz.com
5018 C:
5019 C: Bill:
5020 C: The next meeting of the board of directors will be
5021 C: on Tuesday.
5022 C: John.
5023 C: .
5024 S: 250 OK
5025 C: QUIT
5026 S: 221 foo.com Service closing transmission channel
5042 Klensin Standards Track [Page 90]
5043 \f
5044 RFC 5321 SMTP October 2008
5047 Step 2 -- Relay Host to Destination Host
5049 foo.com, having received the message, now does a DNS lookup on
5050 xyz.com. It finds the same set of MX records, but cannot use the one
5051 that points to itself (or to any other host as a worse preference).
5052 It tries to open a connection to xyz.com itself and succeeds. Then
5053 we have:
5055 S: 220 xyz.com Simple Mail Transfer Service Ready
5056 C: EHLO foo.com
5057 S: 250 xyz.com is on the air
5058 C: MAIL FROM:<JQP@bar.com>
5059 S: 250 OK
5060 C: RCPT TO:<Jones@XYZ.COM>
5061 S: 250 OK
5062 C: DATA
5063 S: 354 Start mail input; end with <CRLF>.<CRLF>
5064 C: Received: from bar.com by foo.com ; Thu, 21 May 1998
5065 C: 05:33:29 -0700
5066 C: Date: Thu, 21 May 1998 05:33:22 -0700
5067 C: From: John Q. Public <JQP@bar.com>
5068 C: Subject: The Next Meeting of the Board
5069 C: To: Jones@xyz.com
5070 C:
5071 C: Bill:
5072 C: The next meeting of the board of directors will be
5073 C: on Tuesday.
5074 C: John.
5075 C: .
5076 S: 250 OK
5077 C: QUIT
5078 S: 221 foo.com Service closing transmission channel
5098 Klensin Standards Track [Page 91]
5099 \f
5100 RFC 5321 SMTP October 2008
5103 D.4. Verifying and Sending Scenario
5105 S: 220 foo.com Simple Mail Transfer Service Ready
5106 C: EHLO bar.com
5107 S: 250-foo.com greets bar.com
5108 S: 250-8BITMIME
5109 S: 250-SIZE
5110 S: 250-DSN
5111 S: 250-VRFY
5112 S: 250 HELP
5113 C: VRFY Crispin
5114 S: 250 Mark Crispin <Admin.MRC@foo.com>
5115 C: MAIL FROM:<EAK@bar.com>
5116 S: 250 OK
5117 C: RCPT TO:<Admin.MRC@foo.com>
5118 S: 250 OK
5119 C: DATA
5120 S: 354 Start mail input; end with <CRLF>.<CRLF>
5121 C: Blah blah blah...
5122 C: ...etc. etc. etc.
5123 C: .
5124 S: 250 OK
5125 C: QUIT
5126 S: 221 foo.com Service closing transmission channel
5128 Appendix E. Other Gateway Issues
5130 In general, gateways between the Internet and other mail systems
5131 SHOULD attempt to preserve any layering semantics across the
5132 boundaries between the two mail systems involved. Gateway-
5133 translation approaches that attempt to take shortcuts by mapping
5134 (such as mapping envelope information from one system to the message
5135 header section or body of another) have generally proven to be
5136 inadequate in important ways. Systems translating between
5137 environments that do not support both envelopes and a header section
5138 and Internet mail must be written with the understanding that some
5139 information loss is almost inevitable.
5154 Klensin Standards Track [Page 92]
5155 \f
5156 RFC 5321 SMTP October 2008
5159 Appendix F. Deprecated Features of RFC 821
5161 A few features of RFC 821 have proven to be problematic and SHOULD
5162 NOT be used in Internet mail.
5164 F.1. TURN
5166 This command, described in RFC 821, raises important security issues
5167 since, in the absence of strong authentication of the host requesting
5168 that the client and server switch roles, it can easily be used to
5169 divert mail from its correct destination. Its use is deprecated;
5170 SMTP systems SHOULD NOT use it unless the server can authenticate the
5171 client.
5173 F.2. Source Routing
5175 RFC 821 utilized the concept of explicit source routing to get mail
5176 from one host to another via a series of relays. The requirement to
5177 utilize source routes in regular mail traffic was eliminated by the
5178 introduction of the domain name system "MX" record and the last
5179 significant justification for them was eliminated by the
5180 introduction, in RFC 1123, of a clear requirement that addresses
5181 following an "@" must all be fully-qualified domain names.
5182 Consequently, the only remaining justifications for the use of source
5183 routes are support for very old SMTP clients or MUAs and in mail
5184 system debugging. They can, however, still be useful in the latter
5185 circumstance and for routing mail around serious, but temporary,
5186 problems such as problems with the relevant DNS records.
5188 SMTP servers MUST continue to accept source route syntax as specified
5189 in the main body of this document and in RFC 1123. They MAY, if
5190 necessary, ignore the routes and utilize only the target domain in
5191 the address. If they do utilize the source route, the message MUST
5192 be sent to the first domain shown in the address. In particular, a
5193 server MUST NOT guess at shortcuts within the source route.
5195 Clients SHOULD NOT utilize explicit source routing except under
5196 unusual circumstances, such as debugging or potentially relaying
5197 around firewall or mail system configuration errors.
5199 F.3. HELO
5201 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather
5202 than HELO when the server will accept the former. Servers MUST
5203 continue to accept and process HELO in order to support older
5204 clients.
5210 Klensin Standards Track [Page 93]
5211 \f
5212 RFC 5321 SMTP October 2008
5215 F.4. #-literals
5217 RFC 821 provided for specifying an Internet address as a decimal
5218 integer host number prefixed by a pound sign, "#". In practice, that
5219 form has been obsolete since the introduction of TCP/IP. It is
5220 deprecated and MUST NOT be used.
5222 F.5. Dates and Years
5224 When dates are inserted into messages by SMTP clients or servers
5225 (e.g., in trace header fields), four-digit years MUST BE used. Two-
5226 digit years are deprecated; three-digit years were never permitted in
5227 the Internet mail system.
5229 F.6. Sending versus Mailing
5231 In addition to specifying a mechanism for delivering messages to
5232 user's mailboxes, RFC 821 provided additional, optional, commands to
5233 deliver messages directly to the user's terminal screen. These
5234 commands (SEND, SAML, SOML) were rarely implemented, and changes in
5235 workstation technology and the introduction of other protocols may
5236 have rendered them obsolete even where they are implemented.
5238 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers
5239 MAY implement them. If they are implemented by servers, the
5240 implementation model specified in RFC 821 MUST be used and the
5241 command names MUST be published in the response to the EHLO command.
5243 Author's Address
5245 John C. Klensin
5246 1770 Massachusetts Ave, Suite 322
5247 Cambridge, MA 02140
5248 USA
5250 EMail: john+smtp@jck.com
5266 Klensin Standards Track [Page 94]
5267 \f
5268 RFC 5321 SMTP October 2008
5271 Full Copyright Statement
5273 Copyright (C) The IETF Trust (2008).
5275 This document is subject to the rights, licenses and restrictions
5276 contained in BCP 78, and except as set forth therein, the authors
5277 retain all their rights.
5279 This document and the information contained herein are provided on an
5280 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
5281 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
5282 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
5283 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
5284 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
5285 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
5287 Intellectual Property
5289 The IETF takes no position regarding the validity or scope of any
5290 Intellectual Property Rights or other rights that might be claimed to
5291 pertain to the implementation or use of the technology described in
5292 this document or the extent to which any license under such rights
5293 might or might not be available; nor does it represent that it has
5294 made any independent effort to identify any such rights. Information
5295 on the procedures with respect to rights in RFC documents can be
5296 found in BCP 78 and BCP 79.
5298 Copies of IPR disclosures made to the IETF Secretariat and any
5299 assurances of licenses to be made available, or the result of an
5300 attempt made to obtain a general license or permission for the use of
5301 such proprietary rights by implementers or users of this
5302 specification can be obtained from the IETF on-line IPR repository at
5303 http://www.ietf.org/ipr.
5305 The IETF invites any interested party to bring to its attention any
5306 copyrights, patents or patent applications, or other proprietary
5307 rights that may cover technology that may be required to implement
5308 this standard. Please address the information to the IETF at
5309 ietf-ipr@ietf.org.
5322 Klensin Standards Track [Page 95]
5323 \f