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17 Architecture Overview </h1>
19 <hr>
23 <p> This document presents an overview of the Postfix architecture,
24 and provides pointers to descriptions of every Postfix command
25 or server program. The text gives the general context in which
26 each command or server program is used, and provides pointers to
27 documents with specific usage examples and background information.
28 </p>
32 <ul>
42 </ul>
46 <p> When a message enters the Postfix mail system, the first stop
47 on the inside is the incoming queue. The figure below shows the
48 main processes that are involved with new mail. Names followed by
49 a number are Postfix commands or server programs, while unnumbered
50 names inside shaded areas represent Postfix queues. </p>
52 <blockquote>
54 <table>
56 <tr>
62 </tr>
64 <tr>
69 </td>
71 <td> </td>
75 </td> </tr> </table> </td>
77 </tr>
79 <tr>
83 </tr>
85 <tr>
90 </td>
95 </td>
102 </tr>
104 <tr>
108 </tr>
110 <tr>
115 </td>
122 </tr>
124 <tr>
130 </tr>
132 <tr>
137 </td>
142 </td>
144 </tr>
146 </table>
148 </blockquote>
150 <ul>
153 servers. These servers remove the SMTP or QMQP protocol encapsulation,
154 enforce some sanity checks to protect Postfix, and give the sender,
155 recipients and message content to the cleanup(8) server. The
156 smtpd(8) server can be configured to block unwanted mail, as
157 described in the SMTPD_ACCESS_README document. </p>
160 compatibility command, and are queued in the maildrop queue by
161 the privileged postdrop(1) command. This arrangement even works
162 while the Postfix mail system is not running. The local pickup(8)
163 server picks up local submissions, enforces some sanity checks to
164 protect Postfix, and gives the sender, recipients and message
167 <li> <p> Mail from internal sources is given directly to the
168 cleanup(8) server. These sources are not shown in the figure, and
169 include: mail that is forwarded by the local(8) delivery agent (see
170 next section), messages that are returned to the sender by the
171 bounce(8) server (see second-next section), and postmaster
172 notifications about problems with Postfix. </p>
175 stage before mail is queued. It adds missing From: and other message
176 headers, and transforms addresses as described in the
177 ADDRESS_REWRITING_README
178 document. Optionally, the cleanup(8) server can be configured to
179 do light-weight content inspection with regular expressions as
180 described in the BUILTIN_FILTER_README document. The cleanup(8)
181 server places the result as a single file into the incoming queue,
182 and notifies the queue manager (see next section) of the arrival
183 of new mail. </p>
186 standard "user@fully.qualified.domain" form, as described in the
187 ADDRESS_REWRITING_README document. Postfix currently does not
188 implement a rewriting language, but a lot can be done via table
189 lookups and, if need be, regular expressions. </p>
191 </ul>
195 <p> Once a message has reached the incoming queue the next step is
196 to deliver it. The figure shows the main components of the Postfix
197 mail delivery apparatus. Names followed by a number are Postfix
198 commands or server programs, while unnumbered names inside shaded
199 areas represent Postfix queues. </p>
201 <blockquote>
203 <table>
205 <tr>
210 </td>
212 <td> </td>
218 </tr>
220 <tr>
224 </tr>
226 <tr>
232 </td> </tr> </table> </td>
240 </tr>
242 <tr>
246 </tr>
248 <tr>
268 </tr>
270 <tr>
276 </td> </tr> </table> </td>
282 </tr>
284 <tr>
292 </tr>
294 <tr>
305 </tr>
307 <tr>
315 </tr>
317 </table>
319 </blockquote>
321 <ul>
324 figure) is the heart of Postfix mail delivery. It contacts the
326 error(8) delivery agents, and sends a delivery request for one
328 agents are special: they discard or bounce all mail, and are not
329 shown in the figure above. </p>
331 <p> The queue manager maintains a small active queue with the
332 messages that it has opened for delivery. The active queue acts as
333 a limited window on potentially large incoming or deferred queues.
334 The limited active queue prevents the queue manager from running
335 out of memory under heavy load. </p>
337 <p> The queue manager maintains a separate deferred queue for mail
338 that cannot be delivered, so that a large mail backlog will not
339 slow down normal queue accesses. The queue manager's strategy for
340 delayed mail delivery attempts is described in the QSHAPE_README
341 and TUNING_README documents. </p>
344 address according to its local or remote address class, as defined
345 in the ADDRESS_CLASS_README document. Additional routing information
346 can be specified with the optional transport(5) table. The
348 for recipients whose address has changed; mail for such recipients is
349 returned to the sender with an explanation. </p>
352 the destination host, sorts the list by preference, and tries each
353 server in turn until it finds a server that responds. It then
354 encapsulates the sender, recipient and message content as required
355 by the SMTP protocol; this includes conversion of 8-bit MIME to
359 is optimized for delivery to mailbox servers such as Cyrus. The
360 advantage of this setup is that one Postfix machine can feed multiple
361 mailbox servers over LMTP. The opposite is true as well: one
362 mailbox server can be fed over LMTP by multiple Postfix machines.
363 </p>
366 qmail-compatible maildir files, Sendmail-style system-wide aliases(5)
367 databases, and Sendmail-style per-user .forward files. Multiple
368 local delivery agents can be run in parallel, but parallel delivery
369 to the same user is usually limited. </p>
372 local delivery: you can configure it to deliver to mailbox files
373 in user home directories, you can configure it to delegate mailbox
374 delivery to an external command such as procmail, or you can delegate
375 delivery to a different Postfix delivery agent. </p>
378 agent that delivers to UNIX-style mailbox or qmail-style maildir
379 files only. This delivery agent can deliver mail for multiple
380 domains, which makes it especially suitable for hosting lots of
381 small domains on a single machine. This is described in the
382 VIRTUAL_README document. </p>
385 processing systems (the Postfix sendmail(1) command being the
386 inbound interface). The interface is UNIX compatible: it provides
387 information on the command line and on the standard input stream,
389 Examples of delivery via the pipe(8) mailer are in the MAILDROP_README
390 and UUCP_README documents.
392 </ul>
396 <p> The previous sections gave an overview of how Postfix server
397 processes send and receive mail. These server processes rely on
398 other server processes that do things behind the scenes. The text
399 below attempts to visualize each service in its own context. As
400 before, names followed by a number are Postfix commands or server
401 programs, while unnumbered names inside shaded areas represent
402 Postfix queues. </p>
404 <ul>
407 an eye on the well-being of the Postfix mail system. It is typically
408 started at system boot time with the "postfix start" command, and
409 keeps running until the system goes down. The master(8) server is
410 responsible for starting Postfix server processes to receive and
411 deliver mail, and for restarting servers that terminate prematurely
412 because of some problem. The master(8) server is also responsible
413 for enforcing the server process count limits as specified in the
415 program hierarchy when Postfix is started up. Only some of the mail
416 handling daemon processes are shown. </p>
418 <table>
446 </table>
449 request rate
450 limiting for all smtpd(8) servers. The TUNING_README document
451 provides guidance for dealing with mis-behaving SMTP clients. The
453 </p>
455 <table>
460 </td> </tr>
462 </table>
465 their own queue directory trees with per-message logfiles. Postfix
467 delivery status notifications to the sender. </p>
470 "sendmail
472 Postfix delivers mail, and is available with Postfix version 2.1
476 <table>
501 </table>
504 implement both ETRN and "sendmail -qRdestination", as described
505 in the ETRN_README document. This moves selected queue files from
506 the deferred queue back to the incoming queue and requests their
507 delivery. The flush(8) service is available with Postfix version
510 <table>
515 </td> </tr>
534 </table>
537 table lookup
538 service to Postfix processes. This overcomes chroot restrictions,
539 reduces the number of open lookup tables by sharing one open
540 table among multiple processes, and implements single-updater
541 tables. </p>
544 the Postfix smtp(8) client. When connection caching is enabled for
545 selected destinations, the smtp(8) client does not disconnect
546 immediately after a mail transaction, but gives the connection to
547 the connection cache server which keeps the connection open for a
548 limited amount of time. The smtp(8) client continues with some
549 other mail delivery request. Meanwhile, any smtp(8) process can
550 ask the scache(8) server for that cached connection and reuse it
551 for mail delivery. As a safety measure, Postfix limits the number
552 of times that a connection may be reused. </p>
554 <p> When delivering mail to a destination with multiple mail servers,
555 connection caching can help to skip over a non-responding server,
556 and thus dramatically speed up delivery. SMTP connection caching
557 is available in Postfix version 2.2 and later. More information
558 about this feature is in the CONNECTION_CACHE_README document. </p>
560 <table>
569 </tr>
581 </table>
584 is the queue listing service that does the work for the mailq(1)
587 <table>
596 </td> </tr>
598 </table>
601 with the client connected via socket or FIFO to the command's
602 standard input, output and error streams. You can find examples of
603 its use in the SMTPD_POLICY_README document. </p>
606 Security, formerly known as SSL) is turned on in the Postfix smtp(8)
609 <ul>
611 <li> <p> Maintain the pseudo-random number generator (PRNG) that
613 server processes. The state of this PRNG is periodically saved to
617 server caches with TLS session keys. Saved keys can improve
618 performance by reducing the amount of computation at the start of
619 a TLS session. </p>
621 </ul>
624 Information about the Postfix TLS implementation is in the TLS_README
625 document. </p>
627 <table>
649 </table>
653 address is deliverable before the smtpd(8) server accepts it. The
654 verify(8) server injects probe messages into the Postfix queue and
655 processes status updates from delivery agents and/or queue manager.
656 This process is described in the ADDRESS_VERIFICATION_README
657 document. The verify(8) service is available with Postfix version
660 <table>
669 </tr>
677 </td> </tr>
679 </table>
681 </ul>
685 <p> The Postfix architecture overview ends with a summary of
686 command-line utilities for day-to-day use of the Postfix mail
688 newaliases(1) commands, the Postfix system comes with it own
689 collection of command-line utilities. For consistency, these are
692 <ul>
695 system. It is the interface for starting, stopping, and restarting
696 the mail system, as well as for some other administrative operations.
697 This command is reserved to the super-user. </p>
700 databases. This is the program that does the work for the
704 queue files. This is a limited, preliminary utility. This program
705 is likely to be superseded by something more powerful that can also
706 edit Postfix queue files. </p>
709 parameters and displays system dependent information about the
710 supported file locking methods, and the supported types of lookup
711 tables. </p>
714 is run by the Postfix sendmail(1) command in order to deposit mail
715 into the maildrop queue directory. </p>
718 communication channels available for use in, for example, shell
719 scripts. </p>
722 locking for use in, for example, shell scripts. </p>
725 for shell scripts. </p>
729 UNIX makemap command. </p>
732 command for each Postfix instance, and supports creation, deletion
733 etc. of Postfix instances. For a tutorial, see MULTI_INSTANCE_README.
734 </p>
738 list the
739 mail queue. </p>
742 removes old temporary files, and moves queue files into the right
743 directory after a change in the hashing depth of queue directories.
744 This command is run at mail system startup time and when Postfix
745 is restarted. </p>
747 </ul>
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