SpamAssassins

SpamAssassins is an open-source email filtering program used to identify and quarantine or flag unsolicited email. It operates by applying a large set of tests to messages, assigning each test a score, and computing a total spam score for the message. If the score surpasses a configured threshold, the message is treated as spam; otherwise it is delivered to the recipient’s inbox.

SpamAssassins uses a modular architecture that can be integrated with Mail Transfer Agents such as Postfix, Exim, and Sendmail. The core components include a daemon (spamd) that runs the analysis and a client (spamc) used by mail delivery systems to request filtering. It relies on a rules-based engine with thousands of rules that inspect headers, content, and message footprints, as well as probabilistic filtering such as a Bayesian classifier. It can also leverage external feeds like DNS-based blacklists and URL blacklists to improve detection.

Configuration is text-based and supports site-wide and per-user settings. Features include white/black lists, automatic learning via Bayes, and various scoring policies. SpamAssassins is written in Perl and is distributed under an open-source license (Apache License 2.0). It is maintained by the community under the auspices of the Apache Software Foundation, and has seen widespread use across mail servers and hosting providers.

History: SpamAssassins originated as a community project to combat spam and grew into a widely deployed solution with regular rule updates and plugin support.