Greylite is a SPAM filter with exceptional effectiveness and without false
positives. It combines natively with qmail and works as a proxy for any SMTP
server.
 
It implements a modified greylisting algorithm that improves the filtering
effectiveness and minimizes the delay drawbacks associated with the standard
greylisting algorithm.

It can be tuned to recognize suspicious clients and reject their attempts
multiple times, reaching filtering rates of over 99% without false positives.

Greylite is easy to setup and maintain, and it is small and fast. 

WWW: http://mij.oltrelinux.com/net/greylite/