Anti-Spam SMTP Proxy

The Anti-Spam SMTP Proxy (ASSP) is an open-source, Perl based, platform-independent transparent SMTP proxy server.

Features
Some ASSP's features are:
 * Hidden Markov Model spam filtering
 * Bayesian spam filtering
 * whitelisting
 * Penalty Box (PB) trapping
 * DNSBL/RBL (Realtime Blackhole Listing)
 * URIBL (Uniform Resource Identifier Black Listing)
 * Multi-level SPF (Sender Policy Framework) validation and blocking
 * DKIM signing and validation
 * DMARC validation and reporting
 * SRS (Sender Rewriting Scheme) fix-up
 * Session Delaying/Greylisting and connection response delaying
 * Sender validation and recipient validation
 * Multi-level attachment blocking (based on block lists or allow lists or content based executable blocking)
 * As well as multiple RFC validation mechanisms.
 * Multi-threaded (Since version 2.x)
 * Platform independent (Written in Perl)