From Ars Digital:
Spam detection and tagging for mail servers. SpamWarn consists of a server that performs Bayesian analysis of e-mail on behalf of a client, and a proxy which sits on either the SMTP or POP3 sides of typical e-mail servers and submits e-mail that it traps to the server, therefore acting as its client. SpamWarn will analyze e-mail and add proper headers and a prefix to the Subject indicating what type of e-mail it has received ("spam: " is added to the subject if e-mail is spam). Users can then define filters locally to handle spam. The system works on a per-user basis so that training of the system can be done for a user's particular type of e-mail.