SpamAssassin is a computer program used for e-mail spam filtering based on content-matching rules. SpamAssassin uses a variety of spam-detection techniques, that includes DNS-based and 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.