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Anti-Spam SMTP Proxy Server

Anti-Spam SMTP Proxy Server (ASSP)

ASSP is an open source, Perl based transparent SMTP proxy server. The spam filter sits on port 25 (SMTP), and optionally also port 465 (SMTPS) and port 587 (Submission), in front of a SMTP server. 

ASSP relays the SMTP dialog between an incoming client and your SMTP server, intercepting the dialog as needed. ASSP performs a number of configurable spam checks and on detecting a spam message provides an immediate 5xx SMTP error code back to the client. Non-spam messages are passed to your regular SMTP serverfor further processing and delivery. Spam messages can be blocked from delivery or subject-tagged and delivered.

This software is easy to set up, requiring only minimal changes to the configuration of the Mail Transfer Agent of the system.

 ASSP 2.4.3

Free to download



Fritz Borgstedt and others


System Requirements
Perl 5.8 or higher
Perl Modules:
Compress::Zlib, Digest::MD5 Digest::SHA1 Email::MIME, Email::MIME::Modifier Email::Send, Email::Valid File::ReadBackwards File::Scan::ClamAV IO::Socket::SSL, LWP::Simple Mail::SPF Mail::SRS Net::CIDR::Lite, Net::DNS Net::IP::Match::Regexp, Net::LDAP Net::SMTP, Net::SenderBase Net::Syslog Sys::Syslog, Time::HiRes

Wiki, Forums, Mailing Lists

Selected Reviews:

Features include:

  • Easy browser-based setup
  • Self learning Bayesian analysis which intelligently classifies email into spam and non-spam
  • Weighted Regular Expression Filtering
  • Penalty Box (PB) trapping
  • Realtime Blackhole Listing DNSBL/RBL
  • URIBL (Uniform Resource Identifier Black Listing)
  • Multi-level SPF (Sender Policy Framework) validation and blocking
  • SRS (Sender Rewriting Scheme) fix-up
  • Session Delaying/Greylisting and connection response delaying
  • Automatic whitelist
  • Sender validation and recipient validation (LDAP)
  • Multi-level attachment blocking (based on block lists or allow lists)
  • Virus scanning (ClamAV and FileScan)
  • Senderbase and multiple other filter methods
  • Multiple RFC validation mechanisms
  • Native SSL-support
  • Multiple Weighted DNSBLs
  • Multiple Weighted URIBLs
  • Backscatter Detection
  • Works with Sendmail, qmail, Postfix, Imail, Hmailserver, Kerio, Exchange, Courier, Mercury, Lotus Notes, and all other standard SMTP servers
  • Uses honeypot type spambucket addresses to automatically recognize spam and update your spam database

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Last Updated Monday, April 20 2015 @ 02:43 PM EDT

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