SpamAssassin

SpamAssassin is a computer program released under the Apache License 2.0 used for e-mail spam filtering based on content-matching rules. It is now part of the Apache Foundation.

SpamAssassin uses a variety of spam-detection techniques, that includes DNS-based and fuzzy-checksum-based spam detection, Bayesian filtering, external programs, blacklists and online databases.

The program can be integrated with the mail server to automatically filter all mail for a site. It can also be run by individual users on their own mailbox and integrates with several mail programs. SpamAssassin is highly configurable; if used as a system-wide filter it can still be configured to support per-user preferences.

SpamAssassin was awarded the Linux New Media Award 2006 as the "Best Linux-based Anti-spam Solution".

Read more about SpamAssassin:  History, Methods of Usage, Operation, Network-based Filtering Methods, Bayesian Filtering, Licensing, Sa-compile, Testing SpamAssassin