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)