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===Main Postfix built-in features===
===Main Postfix built-in features===
* Standards-compliant support for [[Simple Mail Transfer Protocol|SMTP]], [[Local Mail Transfer Protocol|LMTP]], [[STARTTLS]] encryption, [[Simple Authentication and Security Layer|SASL]] authentication, [[Multipurpose Internet Mail Extensions|MIME]] encapsulation and transformation, [[Non delivery report|DSN]] delivery status notifications, [[IPv4]], and [[IPv6]]
* Standards-compliant support for [[Local Mail Transfer Protocol|LMTP]], [[STARTTLS]] encryption, [[Simple Authentication and Security Layer|SASL]] authentication, [[Multipurpose Internet Mail Extensions|MIME]] encapsulation and transformation, [[Non delivery report|DSN]] delivery status notifications, [[IPv4]], and [[IPv6]]
* Configurable [[Simple Mail Transfer Protocol|SMTP]]-level access policy that automatically adapts to overload
* Configurable [[Simple Mail Transfer Protocol|SMTP]]-level access policy that automatically adapts to overload
* "Virtual" domains with distinct address-namespaces
* "Virtual" domains with distinct address-namespaces

Revision as of 01:28, 17 May 2013

This article is about the Mail Transfer Agent. For other uses, see Postfix (disambiguation).
Postfix
Developer(s)Wietse Venema and many others
Stable release
2.10.0 / February 11, 2013 (2013-02-11)
Preview release
2.11-20130512 / May 12, 2013 (2013-05-12)
Repository
Written inC[1]
Operating systemCross-platform
TypeMail transfer agent
LicenseIBM Public License
Websitewww.postfix.org

In computing, Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail, intended as an alternative to the widely used Sendmail MTA.

Postfix is released under the IBM Public License 1.0 which is a free software licence.

Originally written in 1997 by Wietse Venema at the IBM Thomas J. Watson Research Center and first released in December 1998, Postfix continues as of 2013 to be actively developed by its creator and other contributors. The software is also known by its former names VMailer and IBM Secure Mailer.

In January 2012 in a study performed by E-Soft, Inc.,[2] approximately 23% of the publicly reachable mail-servers on the Internet ran Postfix.

Typical deployment

As an SMTP server, Postfix implements a first layer of defense against spambots and malware. Postfix is often combined with other software, for spam/virus filtering (e.g., Amavisd-new), for message store access (e.g., Dovecot), and perhaps for complex SMTP-level access policies (e.g., postfwd, policyd-weight or greylisting).

As an SMTP client, Postfix implements a high-performance parallelized mail delivery engine. Here, Postfix is often combined with mailing list software (e.g., Mailman).

Features

Postfix implements a limited number of features in the MTA itself, and relies on third-party extensions for the rest.

Main Postfix built-in features

  • Standards-compliant support for LMTP, STARTTLS encryption, SASL authentication, MIME encapsulation and transformation, DSN delivery status notifications, IPv4, and IPv6
  • Configurable SMTP-level access policy that automatically adapts to overload
  • "Virtual" domains with distinct address-namespaces
  • UNIX-system interfaces for command-line submission, for delivery to command, and for direct delivery to message stores in mbox and maildir format
  • Light-weight content inspection based on regular expressions
  • A large number of database lookup mechanisms including Berkeley DB, CDB, Memcached, LDAP and multiple SQL database implementations
  • A sophisticated scheduler that implements parallel deliveries, with configurable concurrency and back-off strategies
  • A scalable zombie blocker that reduces the server load due to botnet spam

Typical Postfix extension features

Postfix extensions use the SMTP or Milter (Sendmail mail filter) protocols which both give full control over the message envelope and content, or a simple text-based protocol that enables complex SMTP-level access control policies.

  • Deep content inspection before or after a message is accepted into the mail queue;
  • Mail authentication with DKIM, SPF, or other protocols;
  • SMTP-level access policies such as greylisting or rate control.

Operating systems

Postfix runs on AIX, BSD, HP-UX, IRIX, GNU/Linux, Mac OS X, Solaris, Tru64 UNIX and, generally speaking, on every Unix-like operating system that ships with a C compiler and delivers a standard POSIX development environment. It is the default MTA for the Mac OS X, NetBSD[3] and Ubuntu operating systems.[4]

Architecture

The Postfix core consists of several dozen daemon programs that run in the background, each handling one specific aspect of email delivery. Examples are the SMTP server, the scheduler, the address rewriter, and the local delivery daemon. For damage-control purposes, most daemon programs run with fixed reduced privileges, and terminate voluntarily after processing a limited number of requests. Only a small number of Postfix daemons interact with users through two small "gate" programs for mail submission and for queue status information.

Implementation

The Postfix implementation uses safe subsets of the C language and of the POSIX system API. These subsets are buried under an abstraction layer that contains about 50% of all Postfix source code, and that provides the foundation on which all Postfix programs are built. For example, the "vstring" primitive makes Postfix code resistant to buffer overflow[5] attacks, and the "safe open" primitive makes Postfix code resistant to race condition attacks on systems that implement the POSIX file system API. This abstraction layer does not affect the attack resistance of non-Postfix code, such as code in system libraries or in third-party libraries.

Robustness

Conceptually, Postfix manages pipelines of processes that pass the responsibility for message delivery and error notification from one process to the next. All message and notification "state" information is persisted in the file system. The processes in a pipeline operate mostly without centralized control; this relative autonomy simplifies error recovery. When a process fails before completing its part of a file or protocol transaction, its predecessor in the pipeline backs off and retries the request later, and its successor in the pipeline discards unfinished work. Many Postfix daemons can simply "die" when they run into a problem; they are automatically restarted when the next service request arrives. This approach makes Postfix highly resilient, as long as the operating system or hardware don't fail catastrophically.

Performance

Postfix has been clocked at ~300 message deliveries/second[6] across the Internet, running on commodity hardware (a vintage-2003 Dell 1850 system with battery-backed MegaRAID controller and two SCSI disks). This delivery rate is an order of magnitude below the "intrinsic" limit of 2500 message deliveries/second[6] that was achieved with the mail queue on a RAM disk while delivering to the "discard" transport (with a dual-core Opteron system in 2007).

Mail systems such as Postfix and Qmail achieve high performance by delivering mail in parallel sessions. With mail systems such as Sendmail and Exim that make one connection at a time, high performance can be achieved by submitting limited batches of mail in parallel, so that each batch is delivered by a different process. Postfix and Qmail require parallel submission into different MTA instances once they reach their intrinsic performance limit, or the performance limits of the hardware or operating system.

It should be noted that the delivery rates cited above are largely academic. With bulk mail delivery, the true delivery rate is primarily determined by the receiver's policies and by the sender's reputation. High-performance email delivery is predominantly a logistical problem.

Base configuration

The main.cf file stores site specific Postfix configuration parameters while master.cf defines daemon processes. The Postfix Basic Configuration tutorial covers the core settings that each site needs to consider.

The Postfix Standard Configuration Examples document discusses configuration settings for a few common environments.

The Postfix Address Rewriting document covers address rewriting and mail routing. The full documentation collection is at Postfix Documentation

More complex Postfix implementations include integration with (for example) SpamAssassin and support for multiple (virtual) domain names, where data in databases such as MySQL can drive complex configurations.[7]

Release history

Version Release date Significant changes
Major Minor
2.0
2.1
2.2
2.3
2.4
2.5 2.5.0 24 January 2008
2.5.1 18 February 2008
2.5.2 8 March 2008
2.5.3 27 July 2008
2.5.4 14 August 2008
2.5.5 2 September 2008
2.5.6 3 January 2009
2.5.7 12 May 2009
2.5.8 28 August 2009 Withdrawn release.
2.5.9 28 August 2009
2.5.10 23 March 2010
2.5.11 23 November 2010
2.5.12 7 March 2011
2.5.13 9 May 2011
2.5.14 11 July 2011
2.5.15 3 September 2011
2.5.16 24 October 2011
2.5.17 6 February 2012
2.6 2.6.0 12 May 2009
2.6.1 19 May 2009
2.6.2 3 June 2009
2.6.3 2 August 2009 Unannounced release.
2.6.4 26 August 2009 Withdrawn release.
2.6.5 28 August 2009
2.6.6 23 March 2010
2.6.7 8 June 2010
2.6.8 23 November 2010
2.6.9 7 March 2011
2.6.10 9 May 2011
2.6.11 11 July 2011
2.6.12 3 September 2011
2.6.13 24 October 2011
2.6.14 6 February 2012
2.7 2.7.0 13 February 2010
2.7.1 8 June 2010
2.7.2 23 November 2010
2.7.3 7 March 2011
2.7.4 9 May 2011
2.7.5 11 July 2011
2.7.6 3 September 2011
2.7.7 24 October 2011
2.7.8 6 February 2012
2.8 2.8.0 20 January 2011
2.8.1 22 February 2011
2.8.2 21 March 2011
2.8.3 9 May 2011
2.8.4 7 July 2011
2.8.5 3 September 2011
2.8.6 24 October 2011
2.8.7 7 November 2011
2.8.8 1 February 2012
2.8.9 5 March 2012
2.9 2.9.0 1 February 2012
2.9.1 18 February 2012

See also

Further reading

  • Kyle D. Dent (2003). Postfix: The Definitive Guide. O'Reilly Media. ISBN 0-596-00212-2.
  • Ralf Hildebrandt and Patrick Koetter (2005). The book of Postfix: state-of-the-art message transport. No Starch Press. ISBN 1-59327-001-1.

References

  1. ^ Lextrait, Vincent (2010). "The Programming Languages Beacon, v10.3". Retrieved 5 September 2010. {{cite web}}: Unknown parameter |month= ignored (help)
  2. ^ "E-Soft MX survey".
  3. ^ "The NetBSD Guide. Chapter 27. Mail and news". Retrieved 2010-05-10.
  4. ^ "Postfix". Community Documentation, Ubuntu Wiki. Retrieved 2012-03-09.
  5. ^ Hontañón, Ramón J (July 10, 2001). Linux Security. San Francisco: Sybex. p. 166. ISBN 0-7821-2741-X. Retrieved 2011-09-21.
  6. ^ a b "Bulk Mailing Performance". Retrieved 2012-09-09.
  7. ^ Postfix-Tutorial.com: Postfix, Courier/POP, SASL & Spamassassin – with MySQL admin
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