Fast, free and open-source spam filtering system.

Written in C Perl C
Process model event driven pre-forked pool LDA and pre-forked
MTA integration milter, LDA, custom milter, custom (Amavis) LDA
Web interface embedded 3rd party
Languages support full, UTF-8 conversion, lemmatization naïve (ASCII lowercase) naïve
Scripting support Lua API Perl plugins
Licence Apache 2 Apache 2 GPL
Development status very active active abandoned
Mail filtering features
Replies whitelisting
Rules composition
Filtering methods
Regular expressions filtering
DMARC with reports support
Policies white and blacklists
DNS lists
URL DNS lists
Phishing checks advanced with external resources very basic
Custom lists with dynamic reload
Own fuzzy storage
HTML rules own parser many regexp rules
PDF filtering
Statistical methods
Bayes classifier hidden Markov naïve hidden Markov
Bayes autolearn with custom Lua rules by threshold
Bayes window 5 words 1 word 2 words (5 words in SBPH/OSB mode)
Plain files backend
SQLite3 backend
MySQL backend
Redis backend
Neural networks support via libfann