Anti-Spam SMTP Proxy Server

If you're looking for a reliable spam filter that can enhance the security of your SMTP server, the "Anti-Spam SMTP Proxy" (ASSP) might just be what you need. This software application is designed to sit on port 25 (smtp) and, optionally, also on ports 465 (smtps) and 587 (submission) in front of your regular SMTP server.

One of the main benefits of this software is that it intercepts the SMTP dialog between an incoming client and your SMTP server, relaying it through ASSP, which then performs a number of configurable spam checks. If a spam message is detected, ASSP immediately provides a 5xx SMTP error code back to the client. Non-spam messages are passed on to the regular SMTP server for further processing and delivery.

ASSP also offers various other features, including a whitelist of known good senders, Bayesian checks on message headers and contents, local user validation using RFC822 checks, flat lists or LDAP lookup, relay denial, HELO checking, SPF (Sender Policy Framework) checking, DNSBL (DNS Block List) checking using many block list services, delaying of messages from unknown senders, and virus detection.

Overall, this is a comprehensive and powerful tool for filtering out spam and enhancing the security of your SMTP server.