Postfix (software)

Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail.

This article is about the Mail Transfer Agent. For other uses, see Postfix (disambiguation).
Postfix
Original author(s)Wietse Venema at the IBM Thomas J. Watson Research Center
Developer(s)Venema and many others
Initial releaseDecember 14, 1998 (1998-12-14)
Stable release
3.5.9 / January 17, 2021 (2021-01-17)
Preview release
3.6-20210201 / February 1, 2021 (2021-02-01)
Repository
Written inC[1]
Operating systemCross-platform
TypeMail transfer agent
LicenseIBM Public License or Eclipse Public License
Websitewww.postfix.org

It is released under the IBM Public License 1.0 which is a free software license. Alternatively, starting with version 3.2.5, it is available under the Eclipse Public License 2.0 at the user's option.[2]

Originally written in 1997 by Wietse Venema at the IBM Thomas J. Watson Research Center in New York, and first released in December 1998,[3] Postfix continues as of 2020 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 August 2019 a study performed by E-Soft, Inc.,[4] approximately 34% of the publicly reachable mail-servers on the Internet ran Postfix, making it the second most popular mail server behind Exim.

Typical deployment

As an SMTP server, Postfix implements a first layer of defense against spambots and malware. Administrators can combine Postfix with other software that provides spam/virus filtering (e.g., Amavisd-new), message-store access (e.g., Dovecot), or 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. Postfix is often combined with mailing-list software (such as Mailman).

Operating systems

Postfix runs (or has run) on AIX, BSD, HP-UX, Linux, macOS, Solaris 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 macOS, NetBSD,[5] RedHat/CentOS[6] and Ubuntu operating systems.[7]

Architecture

Postfix consists of a combination of server programs that run in the background, and client programs that are invoked by user programs or by system administrators.

The Postfix core consists of several dozen server 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 server. For damage-control purposes, most server programs run with fixed reduced privileges, and terminate voluntarily after processing a limited number of requests. To conserve system resources, most server programs terminate when they become idle.

Client programs run outside the Postfix core. They interact with Postfix server programs through mail delivery instructions in the user's ~/.forward file, and through small "gate" programs to submit mail or to request queue status information.

Other programs provide administrative support to start or stop Postfix, query status information, manipulate the queue, or to examine or update its configuration files.

Yellow ellipses
One of Postfix' many daemons serving exactly one purpose. This split-up into many smaller pieces of software is considered one of the reasons why Postfix is secure and stable.
Blue boxes
The blue boxes represent so-called lookup tables. A lookup table consists of two columns (key and value) containing information used for access control, e-mail routing etc.
Yellow boxes
The yellow boxes are either mail queues or files. In either case, e-mails are stored on persistent media (e.g., a hard disk).
White clouds
The clouds stand for points at which e-mails enter or leave Postfix. For example, smtpd receives mail from other mail servers or users whereas smtp relays mail to other MTAs.

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[8] 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

One single Postfix instance has been clocked at ~300 message deliveries/second[9] 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[9] 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.

The delivery rates cited above are largely theoretical. With bulk mail delivery, the true delivery rate is primarily determined by the receiver's mail receiving policies and by the sender's reputation.

Base configuration

The main.cf file stores site-specific Postfix configuration parameters while master.cf defines daemon processes.[10] The Postfix Basic Configuration tutorial covers the core settings that each site needs to consider, and 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 may include: integration with other applications such as SpamAssassin; support for multiple virtual domain names - and use databases such as MySQL to control complex configurations.

Release history
Version Release date [11] Significant changes
Major Minor
Old version, no longer maintained: 2.0
Old version, no longer maintained: 2.1
Old version, no longer maintained: 2.2
Old version, no longer maintained: 2.3
Old version, no longer maintained: 2.4
Old version, no longer maintained: 2.5 2.5.024 January 2008
2.5.828 August 2009Withdrawn release.
2.5.176 February 2012EOL 2.5
Old version, no longer maintained: 2.6 2.6.012 May 2009
2.6.32 August 2009Unannounced release.
2.6.426 August 2009Withdrawn release.
2.6.194 February 2013EOL 2.6
Old version, no longer maintained: 2.7 2.7.013 February 2010
2.7.1616 January 2014EOL 2.7[12]
Old version, no longer maintained: 2.8 2.8.020 January 2011Introduction of postscreen daemon that sits before the main postfix and acts as a Zombie blocker. Also: DNS whitelisting, SQLite support[13]
2.8.122 February 2011
2.8.221 March 2011
2.8.39 May 2011
2.8.47 July 2011
2.8.53 September 2011
2.8.624 October 2011
2.8.77 November 2011
2.8.81 February 2012
2.8.95 March 2012
2.8.1024 April 2012
2.8.1120 May 2012
2.8.121 August 2012
2.8.1313 December 2012
2.8.144 February 2013
2.8.1522 June 2013
2.8.165 September 2013
2.8.1716 January 2014
2.8.1813 October 2014
2.8.1919 October 2014
2.8.208 February 2015EOL 2.8[14]
Old version, no longer maintained: 2.9 2.9.01 February 2012Memcache support, gradual degradation[15]
2.9.118 February 2012
2.9.224 April 2012
2.9.320 May 2012
2.9.41 August 2012
2.9.513 December 2012
2.9.64 February 2013
2.9.722 June 2013
2.9.85 September 2013
2.9.916 January 2014
2.9.1013 October 2014
2.9.1119 October 2014
2.9.128 February 2015
2.9.1312 April 2015
2.9.1420 July 2015
2.9.1510 October 2015EOL 2.9[16]
Old version, no longer maintained: 2.10 2.10.011 February 2013Support for TLSv1.1 and TLSv1.2[17]
2.10.122 June 2013
2.10.25 September 2013
2.10.316 January 2014
2.10.413 October 2014
2.10.519 October 2014
2.10.68 February 2015
2.10.712 April 2015
2.10.820 July 2015
2.10.910 October 2015
2.10.1015 May 2016EOL 2.10[18]
Old version, no longer maintained: 2.11 2.11.015 January 2014Support for DANE[19][20]
2.11.17 May 2014
2.11.213 October 2014
2.11.319 October 2014
2.11.48 February 2015
2.11.512 April 2015
2.11.620 July 2015
2.11.710 October 2015
2.11.815 May 2016
2.11.91 January 2017
2.11.1013 June 2017
2.11.1127 January 2018
Old version, no longer maintained: 3.0 3.0.08 February 2015Support for internationalized domain names and address localparts as defined in RFC 6530 and related documents, Postfix dynamically-linked libraries and database plugins, operations on multiple lookup tables, pseudo-tables that make simple things easy to implement, and more.[21]
3.0.112 April 2015
3.0.220 July 2015
3.0.310 October 2015
3.0.421 February 2016
3.0.515 May 2016
3.0.628 August 2016
3.0.71 October 2016
3.0.81 January 2017
3.0.1013 June 2017
3.0.1128 October 2017
3.0.1227 January 2018
3.0.1320 May 2018
3.0.1424 November 2018
3.0.1526 February 2019
Old version, no longer maintained: 3.1 3.1.024 February 2016Simplified setup of opportunistic TLS and SMTP server key/certificate management, positive and negative DNS reply TTL support in postscreen, SASL AUTH rate limit in the SMTP server, safety limit on the number of address verify requests, JSON queue listing, destination-independent delivery rate delay, inter alia.[22]
3.1.115 May 2016
3.1.228 August 2016
3.1.31 October 2016
3.1.41 January 2017
3.1.613 June 2017
3.1.728 October 2017
3.1.827 January 2018
3.1.920 May 2018
3.1.1024 November 2018
3.1.1126 February 2019
3.1.1230 March 2019
3.1.1329 June 2019
3.1.1422 September 2019
3.1.153 February 2020
Older version, yet still maintained: 3.2 3.2.028 February 2017Elliptic curve negotiation with OpenSSL ≥ 1.0.2, stored-procedure support for MySQL databases, cidr: table support for if/endif and negation, support for per-client Milter configuration, "PASS" and "STRIP" actions in header/body_checks, and more.[23]
3.2.213 June 2017
3.2.324 September 2017
3.2.428 October 2017
3.2.527 January 2018
3.2.620 May 2018
3.2.724 November 2018
3.2.826 February 2019
3.2.930 March 2019
3.2.1029 June 2019
3.2.1122 September 2019
3.2.123 February 2020
3.2.1312 March 2020
3.2.1418 April 2020
Older version, yet still maintained: 3.3 3.3.021 February 2018Support for legacy release Postfix 2.11 ended[24]
3.3.120 May 2018
3.3.224 November 2018
3.3.326 February 2019
3.3.430 March 2019
3.3.529 June 2019
3.3.622 September 2019
3.3.73 February 2020
3.3.812 March 2020
3.3.918 April 2020
Older version, yet still maintained: 3.4 3.4.027 February 2019Support for legacy release Postfix 3.0 ended[25]
3.4.17 March 2019
3.4.210 March 2019
3.4.310 March 2019One of the Postfix 3.4.2 changes (LINUX5) was missing.
3.4.414 March 2019
3.4.530 March 2019
3.4.629 June 2019
3.4.722 September 2019
3.4.824 November 2019
3.4.93 February 2020
3.4.1012 March 2020
3.4.1118 April 2020
Current stable version: 3.5 3.5.016 March 2020Support for legacy release Postfix 3.1 ended[26]
3.5.118 April 2020
Latest preview version of a future release: 3.619 April 2020
Legend:
Old version
Older version, still maintained
Latest version
Latest preview version
Future release

See also

References
  1. Lextrait, Vincent (July 2010). "The Programming Languages Beacon, v10.3". Archived from the original on 30 May 2012. Retrieved 5 September 2010.
  2. "Postfix stable release 3.2.5, and legacy releases 3.1.8, 3.0.12, and 2.11.11". January 2018. Retrieved 30 January 2018.
  3. Markoff, John (December 1998). "Sharing Software, IBM to Release Mail Program Blueprint". Retrieved 17 September 2017.
  4. "E-Soft MX survey". securityspace.com. E-Soft Inc. 1 August 2019. Retrieved 21 August 2019.
  5. "The NetBSD Guide. Chapter 27. Mail and news". Retrieved 2010-05-10.
  6. "CHAPTER 24. POSTFIX".
  7. "Postfix". Community Documentation, Ubuntu Wiki. Retrieved 2012-03-09.
  8. Hontañón, Ramón J (July 10, 2001). Linux Security. San Francisco: Sybex. p. 166. ISBN 978-0-7821-2741-6. Retrieved 2011-09-21.
  9. "Bulk Mailing Performance". Retrieved 2012-09-09.
  10. Postfix-Tutorial.com: Postfix, Courier/POP, SASL & Spamassassin – with MySQL admin
  11. Postfix Announcements
  12. "Postfix legacy releases 2.10.3, 2.9.9, 2.8.17, and 2.7.16". Postfix.org. Retrieved 2014-01-24.
  13. "Postfix stable release 2.8.0". Postfix.org. Retrieved 2013-09-19.
  14. "Postfix legacy releases 2.11.4, 2.10.6, 2.9.12, and 2.8.20". Postfix.org. Retrieved 2015-04-29.
  15. "Postfix stable release 2.9.0". Postfix.org. Retrieved 2013-09-19.
  16. "Postfix stable release 3.1.0". Postfix.org. Retrieved 2017-11-21.
  17. "Postfix stable release 2.10.0". Postfix.org. Retrieved 2013-09-19.
  18. "Postfix stable release 3.1.4 and legacy releases 3.0.8, 2.11.9". Postfix.org. Retrieved 2017-11-20.
  19. "Postfix 2.11.0-RC2 available with feature-complete DANE support". IETF.org. Retrieved 2015-04-29.
  20. "Postfix stable release 2.11.0". Postfix.org. Retrieved 2014-01-24.
  21. "Postfix stable release 3.0.0". Postfix.org. Retrieved 2015-04-29.
  22. "Postfix stable release 3.1.0". Postfix.org. Retrieved 2016-03-29.
  23. "Postfix stable release 3.2.0". Postfix.org. Retrieved 2017-11-21.
  24. "Postfix stable release 3.3.0". Postfix.org. Retrieved 2018-02-21.
  25. "Postfix stable release 3.4.0". Postfix.org. Retrieved 2019-02-27.
  26. "Postfix stable release 3.5.0". Postfix.org. Retrieved 2020-03-16.

Further reading
  • Kyle D. Dent (2003). Postfix: The Definitive Guide. O'Reilly Media. ISBN 978-0-596-00212-1.
  • Ralf Hildebrandt and Patrick Koetter (2005). The book of Postfix: state-of-the-art message transport. No Starch Press. ISBN 978-1-59327-001-8.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.