Anti-Spam SMTP Proxy explained

Anti-Spam SMTP Proxy
Author:	John Hanna
Developer:	Thomas Eckardt in association with the ASSP community
Latest Release Version:	2.8.1(23131)
Latest Preview Version:	2.8.2(23136)
Operating System:	Cross-platform
Programming Language:	Perl
Genre:	Email spam filter
License:	GNU General Public License
Website:	assp.sourceforge.net

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)