Apache SpamAssassin is the #1 open source anti-spam platform giving system administrators a filter to classify email and block spam (unsolicited bulk email).

It uses a robust scoring framework and plug-ins to integrate a wide range of advanced heuristic and statistical analysis tests on email headers and body text including text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases.

Features

• Wide-spectrum: SpamAssassin uses a wide variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around.
• Free software: it is distributed under the same terms and conditions as other popular open-source software packages such as the Apache web server.
• Easy to extend: Anti-spam tests and configuration are stored in plain text, making it easy to configure and add new rules.
• Flexible: SpamAssassin encapsulates its logic in a well-designed, abstract API so it can be integrated anywhere in the email stream. The Mail::SpamAssassin classes can be used on a wide variety of email systems including procmail, sendmail, Postfix, qmail, and many others.
• Easy Configuration: SpamAssassin requires very little configuration; you do not need to continually update it with details of your mail accounts, mailing list memberships, etc. Once classified, site and user-specific policies can then be applied against spam. Policies can be applied on both mail servers and later using the user's own mail user-agent application.