SpamAssassin

A popular spam filter used on many email servers.

SpamAssassin works by running each received email through a long list of rules, assigning a point value to the email for each rule. Each of those rules increases or decreases the probability that the email is spam.

The final result is a "spam score", the sum of the scores from all the individual rules. That spam score theoretically indicates the likelyhood of the received email being spam.