Postfix: электрондық пошта тасымалдау агенті

Mail transfer agent
the Mail Transfer Agent

Postfix is a free and open source mail transfer agent (MTA) that routes and delivers electronic mail. 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. Originally written in 1997 by Wietse Venema at the IBM Thomas J. Watson Research Center in New York, and first released in December 1998, Postfix continues as of 2023 to be actively developed by its creator and other contributors. The software is also known by its former names VMailer and IBM Secure Mailer. The name Postfix is a compound of "post" (which is another word for "mail") and "bugfix" (which is for other software that inspired Postfix development).

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, milter regex, policyd weight). As an SMTP client, Postfix implements a high performance parallelized mail delivery engine. Postfix is often combined with mailing list software (such as Mailman).

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. Orange boxes The orange 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.

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.

In Shodan scan results from March 2024, Postfix was detected 2.7 million times, Exim 3.0 million times. Some sites block Shodan probes, but it is unclear whether this would materially affect the Postfix to Exim ratio. In a 10 times smaller study published on March 1, 2024 by E Soft, Inc., 148 thousand mail servers ran Postfix, making it a more distant second behind Exim with 227 thousand. This survey is often cited, but it specifically targets domains that provide web services.