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/