SpamAssassin is a widely-deployed, free and mature open source command-line application that has been designed from the ground up to block spam and unwanted email messages on a Linux/UNIX mail server.
Provides a wide variety of anti-spam mechanisms
It is an easy to extend software, also known as an anti-spam mail filter, that provides a wide variety of anti-spam mechanisms, including text and header analysis, v, Bayesian filtering, as well as collaborative filtering databases.
The software runs entirely in the background, as a daemon, with no GUI (Graphical User Interface) front-end. Users can interact with it via the command-line prompt, using the built-in options.
Numerous command-line options are available for the SpamAssassin program, including the ability to run local tests, to report message as spam, to revoke message as spam, to remove spam reports from a message, to pipe an email through and add an extra report to the bottom of the message, as well as to specify the path to the standard configuration file.
Additionally, is is possible to add an email address to persistent address whitelist or blacklist, to read email messages in mbox or UW mbx format, to remove all email addresses found in a message from a persistent address list, and to disable attempted use of the IPv6 protocol for DNS.
The most powerful email filter for GNU/Linux systems
This is one of the most popular, easy to configure, flexible and powerful email filters for the GNU/Linux operating systems, running on any server, and designed from the offset to stop spam email from reaching a user’s mailbox.
It is written entirely in the Perl programming language, requiring a wide range of Perl modules to work properly, which means that it is best to install it from the main software repositories of your distribution.