DSPAM (as in De-Spam) is an extremely scalable, open-source statistical hybrid
anti-spam filter. While most commercial solutions only provide a mere 95%
accuracy (1 error in 20), a majority of DSPAM users frequently see between
99.95% (1 error in 2000) all the way up to 99.991% (2 errors in 22,786).

DSPAM is currently effective as both a server-side agent for UNIX email servers
and a developer's library for mail clients, other anti-spam tools, and similar
projects requiring drop-in spam filtering. DSPAM has been implemented on many
large and small scale systems with the largest systems being reported at about
125,000 mailboxes.

Homepage:
https://dspam.sourceforge.net/
