SpamAssassin is a computer program used for e-mail Spam filtering based on content-matching rules.
How does it work?
SpamAssassin uses a variety of Spam-detection techniques, including DNS-based and checksum-based Spam detection, Bayesian filtering, external programs, blacklists and online databases.
Who can use it?
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.