spamdyke is a filter for monitoring and intercepting SMTP
connections between a remote host and a qmail server. Spam
is blocked while the remote server (spammer) is still
connected; no additional processing or storage is needed.

In addition to all of its anti-spam filters, spamdyke also
includes a number of features to enhance qmail.

Best of all, using spamdyke does not require patching or
recompiling qmail!

WWW: http://www.spamdyke.org/