Postfix Architecture Overview

Introduction

This document presents an overview of the Postfix architecture, and provides pointers to descriptions of every Postfix command or server program. The text gives the general context in which each command or server program is used, and provides pointers to documents with specific usage examples and background information.

Topics covered by this document:

• How Postfix receives mail
• How Postfix delivers mail
• Postfix behind the scenes
• Postfix support commands

How Postfix receives mail

When a message enters the Postfix mail system, the first stop on the inside is the incoming queue. The figure below shows the main processes that are involved with new mail. Names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues.

				trivial- rewrite(8)
Network	->	smtpd(8)		^ | | v
			\
Network	->	qmqpd(8)	->	cleanup(8)	->	incoming
			/
		pickup(8)	<-	maildrop
				^ |
Local	->	sendmail(1)	->	postdrop(1)

• Network mail enters Postfix via the smtpd(8) or qmqpd(8) servers. These servers remove the SMTP or QMQP protocol encapsulation, enforce some sanity checks to protect Postfix, and give the sender, recipients and message content to the cleanup(8) server. The smtpd(8) server can be configured to block unwanted mail, as described in the SMTPD_ACCESS_README document.

• Local submissions are received with the Postfix sendmail(1) compatibility command, and are queued in the maildrop queue by the privileged postdrop(1) command. This arrangement even works while the Postfix mail system is not running. The local pickup(8) server picks up local submissions, enforces some sanity checks to protect Postfix, and gives the sender, recipients and message content to the cleanup(8) server.

• Mail from internal sources is given directly to the cleanup(8) server. These sources are not shown in the figure, and include: mail that is forwarded by the local(8) delivery agent (see next section), messages that are returned to the sender by the bounce(8) server (see second-next section), and postmaster notifications about problems with Postfix.

• The cleanup(8) server implements the final processing stage before mail is queued. It adds missing From: and other message headers, and transforms addresses as described in the ADDRESS_REWRITING_README document. Optionally, the cleanup(8) server can be configured to do light-weight content inspection with regular expressions as described in the BUILTIN_FILTER_README document. The cleanup(8) server places the result as a single file into the incoming queue, and notifies the queue manager (see next section) of the arrival of new mail.

• The trivial-rewrite(8) server rewrites addresses to the standard "[email protected]" form, as described in the ADDRESS_REWRITING_README document. Postfix currently does not implement a rewriting language, but a lot can be done via table lookups and, if need be, regular expressions.

How Postfix delivers mail

Once a message has reached the incoming queue the next step is to deliver it. The figure shows the main components of the Postfix mail delivery apparatus. Names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues.

				trivial- rewrite(8)		smtp(8)	->	Network
					/
				^ | | v	-	lmtp(8)	->	Network
					/
incoming	->	active	->	qmgr(8)	---	local(8)	->	File, command
		^ | | v			\
					-	virtual(8)	->	File
		deferred			\
						pipe(8)	->	Command

• The queue manager (the qmgr(8) server process in the figure) is the heart of Postfix mail delivery. It contacts the smtp(8), lmtp(8), local(8), virtual(8), pipe(8), discard(8) or error(8) delivery agents, and sends a delivery request for one or more recipient addresses. The discard(8) and error(8) delivery agents are special: they discard or bounce all mail, and are not shown in the figure above.

  The queue manager maintains a small active queue with the messages that it has opened for delivery. The active queue acts as a limited window on potentially large incoming or deferred queues. The limited active queue prevents the queue manager from running out of memory under heavy load.

  The queue manager maintains a separate deferred queue for mail that cannot be delivered, so that a large mail backlog will not slow down normal queue accesses. The queue manager's strategy for delayed mail delivery attempts is described in the QSHAPE_README and TUNING_README documents.

• The trivial-rewrite(8) server resolves each recipient address according to its local or remote address class, as defined in the ADDRESS_CLASS_README document. Additional routing information can be specified with the optional transport(5) table. The trivial-rewrite(8) server optionally queries the relocated(5) table for recipients whose address has changed; mail for such recipients is returned to the sender with an explanation.

• The smtp(8) client looks up a list of mail exchangers for the destination host, sorts the list by preference, and tries each server in turn until it finds a server that responds. It then encapsulates the sender, recipient and message content as required by the SMTP protocol; this includes conversion of 8-bit MIME to 7-bit encoding.

• The lmtp(8) client speaks a protocol similar to SMTP that is optimized for delivery to mailbox servers such as Cyrus. The advantage of this setup is that one Postfix machine can feed multiple mailbox servers over LMTP. The opposite is true as well: one mailbox server can be fed over LMTP by multiple Postfix machines.

• The local(8) delivery agent understands UNIX-style mailboxes, qmail-compatible maildir files, Sendmail-style system-wide aliases(5) databases, and Sendmail-style per-user .forward files. Multiple local delivery agents can be run in parallel, but parallel delivery to the same user is usually limited.

  The local(8) delivery agent has hooks for alternative forms of local delivery: you can configure it to deliver to mailbox files in user home directories, you can configure it to delegate mailbox delivery to an external command such as procmail, or you can delegate delivery to a different Postfix delivery agent.

• The virtual(8) delivery agent is a bare-bones delivery agent that delivers to UNIX-style mailbox or qmail-style maildir files only. This delivery agent can deliver mail for multiple domains, which makes it especially suitable for hosting lots of small domains on a single machine. This is described in the VIRTUAL_README document.

• The pipe(8) mailer is the outbound interface to other mail processing systems (the Postfix sendmail(1) command being the inbound interface). The interface is UNIX compatible: it provides information on the command line and on the standard input stream, and expects a process exit status code as defined in <sysexits.h>. Examples of delivery via the pipe(8) mailer are in the MAILDROP_README and UUCP_README documents.

Postfix behind the scenes

The previous sections gave an overview of how Postfix server processes send and receive mail. These server processes rely on other server processes that do things behind the scenes. The text below attempts to visualize each service in its own context. As before, names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues.

• The resident master(8) server is the supervisor that keeps an eye on the well-being of the Postfix mail system. It is typically started at system boot time with the "postfix start" command, and keeps running until the system goes down. The master(8) server is responsible for starting Postfix server processes to receive and deliver mail, and for restarting servers that terminate prematurely because of some problem. The master(8) server is also responsible for enforcing the server process count limits as specified in the master.cf configuration file. The picture below gives the program hierarchy when Postfix is started up. Only some of the mail handling daemon processes are shown.

  		postfix(1)
  		| |
  		postfix-script(1)
  	/ /	| |	\ \
  postsuper(1)		master(8)		postlog(1)
  	/ /	| |	\ \
  smtpd(8)		qmgr(8)		local(8)

• The anvil(8) server implements client connection and request rate limiting for all smtpd(8) servers. The TUNING_README document provides guidance for dealing with mis-behaving SMTP clients. The anvil(8) service is available in Postfix version 2.2 and later.

  Network

  ->

  smtpd(8)

  <->

  anvil(8)

• The bounce(8), defer(8) and trace(8) services each maintain their own queue directory trees with per-message logfiles. Postfix uses this information when sending "failed", "delayed" or "success" delivery status notifications to the sender.

  The trace(8) service also implements support for the Postfix "sendmail -bv" and "sendmail -v" commands which produce reports about how Postfix delivers mail, and is available with Postfix version 2.1 and later. See DEBUG_README for examples.

  cleanup(8)	->	qmgr(8) Postfix queue	->	Delivery agents
  ^ |		| v		| v
  (Non-) delivery notice	<-	bounce(8) defer(8) trace(8)	<-	Queue id, recipient, status
  		^ | | v
  		Per- message logfiles

• The flush(8) servers maintain per-destination logs and implement both ETRN and "sendmail -qRdestination", as described in the ETRN_README document. This moves selected queue files from the deferred queue back to the incoming queue and requests their delivery. The flush(8) service is available with Postfix version 1.0 and later.

  				incoming ^ deferred
  				^ |
  smtpd(8) sendmail(1) postqueue(1)	-	Destination to flush	->	flush(8)	<-	Deferred destination, queue id	-	Delivery agents, qmgr(8)
  				^ | | v
  				Per-dest- ination logs

• The proxymap(8) servers provide read-only and read-write table lookup service to Postfix processes. This overcomes chroot restrictions, reduces the number of open lookup tables by sharing one open table among multiple processes, and implements single-updater tables.

• The scache(8) server maintains the connection cache for the Postfix smtp(8) client. When connection caching is enabled for selected destinations, the smtp(8) client does not disconnect immediately after a mail transaction, but gives the connection to the connection cache server which keeps the connection open for a limited amount of time. The smtp(8) client continues with some other mail delivery request. Meanwhile, any smtp(8) process can ask the scache(8) server for that cached connection and reuse it for mail delivery. As a safety measure, Postfix limits the number of times that a connection may be reused.

  When delivering mail to a destination with multiple mail servers, connection caching can help to skip over a non-responding server, and thus dramatically speed up delivery. SMTP connection caching is available in Postfix version 2.2 and later. More information about this feature is in the CONNECTION_CACHE_README document.

  	/--	smtp(8)			--> Internet
  qmgr(8)			| | | | v
  	\--			smtp(8)		--> Internet
  				^ |
  			scache(8)

• The showq(8) servers list the Postfix queue status. This is the queue listing service that does the work for the mailq(1) and postqueue(1) commands.

  Output

  <-

  mailq(1) post- queue(1)

  <-

  showq(8)

  <-

  Postfix queue

• The spawn(8) servers run non-Postfix commands on request, with the client connected via socket or FIFO to the command's standard input, output and error streams. You can find examples of its use in the SMTPD_POLICY_README document.

• The tlsmgr(8) server runs when TLS (Transport Layer Security, formerly known as SSL) is turned on in the Postfix smtp(8) client or smtpd(8) server. This process has two duties:

  • Maintain the pseudo-random number generator (PRNG) that is used to seed the TLS engines in Postfix smtp(8) client or smtpd(8) server processes. The state of this PRNG is periodically saved to a file, and is read when tlsmgr(8) starts up.

  • Maintain the optional Postfix smtp(8) client or smtpd(8) server caches with TLS session keys. Saved keys can improve performance by reducing the amount of computation at the start of a TLS session.

  TLS support is available in Postfix version 2.2 and later. Information about the Postfix TLS implementation is in the TLS_README document.

  Network->	smtpd(8)	<---seed--- <-session->		tlsmgr(8)	---seed---> <-session->			smtp(8)	->Network
  			/ /	| |	\ \
  		smtpd session cache		PRNG state file		smtp session cache

• The verify(8) server verifies that a sender or recipient address is deliverable before the smtpd(8) server accepts it. The verify(8) server queries a cache with address verification results. If a result is not found, the verify(8) server injects a probe message into the Postfix queue and processes the status update from a delivery agent or queue manager. This process is described in the ADDRESS_VERIFICATION_README document. The verify(8) service is available with Postfix version 2.1 and later.

  					->	probe message	->	Postfix mail queue
  Network	->	smtpd(8)	<->	verify(8)
  								| v
  					<-	probe status	<-	Postfix delivery agents	-> Local -> Network
  				^ | v
  				Address verification cache

• The postscreen(8) server can be put "in front" of Postfix smtpd(8) processes. Its purpose is to accept connections from the network and to decide what SMTP clients are allowed to talk to Postfix. According to the 2008 MessageLabs annual report, 81% of all email was spam, and 90% of that was sent by botnets; by 2010, those numbers were 92% and 95%, respectively. While postscreen(8) keeps the zombies away, more smtpd(8) processes remain available for legitimate clients.

  postscreen(8) maintains a temporary whitelist for clients that pass its tests; by allowing whitelisted clients to skip tests, postscreen(8) minimizes its impact on legitimate email traffic.

  The postscreen(8) server is available with Postfix 2.8 and later. To keep the implementation simple, postscreen(8) delegates DNS white/blacklist lookups to dnsblog(8) server processes, and delegates TLS encryption/decryption to tlsproxy(8) server processes. This delegation is invisible to the remote SMTP client, and is not shown in the diagram below.

  zombie
  	\
  zombie	-				-	smtpd(8)
  	\		/
  other	---	postscreen(8)
  	/		\
  other	-				-	smtpd(8)
  	/
  zombie

Postfix support commands

The Postfix architecture overview ends with a summary of command-line utilities for day-to-day use of the Postfix mail system. Besides the Sendmail-compatible sendmail(1), mailq(1), and newaliases(1) commands, the Postfix system comes with it own collection of command-line utilities. For consistency, these are all named postsomething.

• The postfix(1) command controls the operation of the mail system. It is the interface for starting, stopping, and restarting the mail system, as well as for some other administrative operations. This command is reserved to the super-user.

• The postalias(1) command maintains Postfix aliases(5) type databases. This is the program that does the work for the newaliases(1) command.

• The postcat(1) command displays the contents of Postfix queue files. This is a limited, preliminary utility. This program is likely to be superseded by something more powerful that can also edit Postfix queue files.

• The postconf(1) command displays or updates Postfix main.cf parameters and displays system dependent information about the supported file locking methods, and the supported types of lookup tables.

• The postdrop(1) command is the mail posting utility that is run by the Postfix sendmail(1) command in order to deposit mail into the maildrop queue directory.

• The postkick(1) command makes some Postfix internal communication channels available for use in, for example, shell scripts.

• The postlock(1) command provides Postfix-compatible mailbox locking for use in, for example, shell scripts.

• The postlog(1) command provides Postfix-compatible logging for shell scripts.

• The postmap(1) command maintains Postfix lookup tables such as canonical(5), virtual(5) and others. It is a cousin of the UNIX makemap command.

• The postmulti(1) command repeats the "postfix start" etc. command for each Postfix instance, and supports creation, deletion etc. of Postfix instances. For a tutorial, see MULTI_INSTANCE_README.

• The postqueue(1) command is the privileged command that is run by Postfix sendmail(1) and mailq(1) in order to flush or list the mail queue.

• The postsuper(1) command maintains the Postfix queue. It removes old temporary files, and moves queue files into the right directory after a change in the hashing depth of queue directories. This command is run at mail system startup time and when Postfix is restarted.