Anti-Spam SMTP Proxy (ASSP) aims to be an Open Source SMTP Proxy server with auto-whitelists, self-learning Bayesian, greylisting, virus scanning, attachment blocking, and many other filter methods.

Some of its other geeky features include multiple weighted DNSBLs and URIBLs, greylisting, regular expression filtering, penalty box, senderBase, attachment blocking, ClamAV and FileScan included, blocking reporting, LDAP support, backscatter detection, and more.

Anti-Spam SMTP Proxy (ASSP) Features:

Multiple Weighted DNSBLs

Multiple Weighted URIBLs

Greylisting

Weighted Regular Expression Filtering

Bayesian

Penalty Box

SenderBase

SSL/TLS

SPF/SRS

Attachment Blocking

ClamAV and FileScan

Blocking Reporting

LDAP support

Backscatter Detection

recipient replacement / GUI user access rights management

MIME charset conversion / DKIM check and signing

Multi DB support for all hashes / level-based open plugin support

Transparent TCP proxy support

Plugins: archive, full attachment check and replacement, OCR

damping (steal spammers' time)

AUTH to relay host / POP3 collector

Configuration value and file synchronization

Block Reports design could be customized

Razor2 and DCC support using Plugin

SNMP support (monitoring, configuring, control-API)

User group import (file or LDAP or command-based)

Automatic crash analyzer Hidden Markov Model

IPv6 socket support

word stemming (several languages) for Bayesian analyzer

Perl module autoupdate via PPM or CPAN

Hidden Markov Model spam detection engine

Full Unicode support

DMARC support

Private Whitelist (legacy, domain-based, user-based)

Private IP and Domain lists (legacy, domain-based, user-based)

CPU affinity settings

Web file commander

Graphical statistic output (SVG)

Native SSL support for target hosts

Client SSL-certificate validation

Private SMIME signing

Corporate SMIME signing, using a single private certificate

Content-based executable attachment blocking

Content-based compressed attachment blocking

BATV check and signing

Message-ID signing and check FBMTV

Remote support

Authenticated Received Chain (ARC) processing and signing

RSS feeds for blocked emails using Plugin

Zero-day virus detection

VirusTotal API