SpamAssassin is a mail filter which attempts to identify spam using text
analysis and several internet-based realtime blacklists.

Using its rule base, it uses a wide range of heuristic tests on mail
headers and body text to identify "spam", also known as unsolicited
commercial email.

Once identified, the mail can then be optionally tagged as spam for later
filtering using the user's own mail user-agent application.

WWW:    http://spamassassin.org/

Basically, SpamAssassin runs emails passed to it through a score of tests and assigns a score based on the tests' results. Then, it compares the score to a configurable threshold to desided whether the email is a spam message or not, and if so, adds a header to the email marking it as such.

SpamAssassin is a tremendously powerful tool in the fight against spam, since users can set up trivial filters in their email clients to look for SpamAssassin's warning headers.

-- KirkStrauser - 04 Feb 2003