DSPAM

DSPAM: "DSPAM (as in De-Spam) is an open-source, freely available anti-spam project. DSPAM effectively filters spam system-wide using advanced techniques combined with one or more of the supported algorithms (Bayesian, Alternative Bayesian, and Chi-Square are currently supported). A majority of users achieve filtering rates ranging from 99.55% - 99.9%, with some users reporting an actual 100% accuracy rate during long periods after initial learning. 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."