Fast Bayesian spam filter along lines suggested by Paul Graham
Anti-Spam SMTP Proxy Server implements multiple spam filters
SMTP email proxy and relay server
Remote anti spam utility
Python API for Sendmail Milters.
An easy-to-use anti-spam email gateway
Open source DMARC implementation
mapSoN is a challenge/response spam filter for Unix
Secure mail + Tor gateway
Protects you from spam mails and virus mails effectively with milter
Blocks spam without blocking any legitimate messages
A Scalable Spam Filtering Architecture
The free cross-platform antivirus software tool-kit