Postfix (software)

Postfix

Developer(s)	Wietse Venema and many others
Initial release	December 1998 (1998-12)
Stable release	3.0.3 / October 10, 2015 (2015-10-10)
Preview release	3.1-20160214 / February 14, 2016 (2016-02-14)
Written in	C[1]
Operating system	Cross-platform
Type	Mail transfer agent
License	IBM Public License
Website	www.postfix.org

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 license.

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 2015 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 November 2015 in a study performed by E-Soft, Inc.,^[2] approximately 32% 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. 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).

Features

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

Main Postfix built-in features

• Standards-compliant support for SMTPUTF8, SMTP, LMTP, STARTTLS encryption including DANE protocol support and "perfect" forward secrecy, 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, OpenLDAP LMDB, 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 SMTP 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 DMARC, DKIM, SPF, or other protocols;
• SMTP-level access policies such as greylisting or rate control.

Operating systems

Postfix runs (or has run) on AIX, BSD, HP-UX, GNU/Linux, OS X, 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 OS X, NetBSD^[3] and Ubuntu operating systems.^[4]

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^[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

One single Postfix instance 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 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.^[7] 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

See also

• List of mail servers
• Comparison of mail servers
• Email filtering

References

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. 

External links

• Official website
• Postfix "how to" with configuration examples and explanation
• Postfix policy delegation server
• Postfix policy delegation server
• Postfix introduction and analysis for secure environments
• #postfix ^connect on freenode
• Postfix Architecture Overview

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