DSPAM is a scalable and open-source content-based spam filter designed for 
multi-user enterprise systems.  On a properly configured system,
many users experience results between 99.5% - 99.95%, or one error for 
every 200 to 2000 messages. DSPAM supports many different MTAs and can also be 
deployed as a stand-alone SMTP appliance. 
For developers, the DSPAM core engine (libdspam) can be easily incorporated 
directly into applications for drop-in filtering

WWW: http://dspam.sourceforge.net/
