TMDA is an open source anti-spam system and local mail delivery agent written in 100% pure Python. TMDA combines whitelists, blacklists, a challenge/response system, and special-purpose e-mail addresses called "tagged addresses" to thwart spam.
You can filter your email using a bayesian like statistical method. Written in 100% Python, easily extensible, and is used as a pop3 proxy to reduce the intrusion on your system.