The Anti-Spam SMTP Proxy (ASSP) server project is an Open Source platform-independent transparent SMTP proxy server that leverages numerous methodologies and technologies to both rigidly and adaptively identify e-mail spam. ASSP is easy to set up because it requires only minor changes to your MTA.

Foremost amongst ASSP's features are: Bayesian analysis, Penalty Box (PB) trapping, RBL (Real-time Black-hole Listing, aka DNSBL), URIBL (aka Uniform Resource Identifier Black Listing), multi-level SPF (aka Sender Policy Framework) validation and blocking, SRS (aka Sender Rewriting Scheme) fix-up, Session Delaying (aka Greylisting) and connection response delaying, sender validation and recipient validation, multi-level attachment blocking (based on block lists or allow lists), as well as multiple RFC validation mechanisms.

The beta version of 1.2.7 now features URL blacklisting which will block email with known spam websites.

ASSP is a project on SourceForge.net.