Anti-Spam SMTP Proxy is a spam filter that sits on port 25 in front of your regular SMTP server (sendmail, postfix, qmail, etc). 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 server for further processing and delivery. ASSP includes SSL and IPv6 support. It is a single script with a web-based configuration tool. ASSP offers: - a whitelist of known good senders - Bayesian checks on message headers and contents - recipient address validation using LDAP and RFC822 conformance - relay denial - HELO checking - SPF (Sender Policy Framework) checking - DNSBL (DNS Block List) checking using many DNSBL services - various SMTP error modes detection - Virus detection and many other spam detection techniques. See also: http://assp.sourceforge.net/