Postpals is a simple policy daemon for Postfix. It keeps a database of 
outgoing mail, specifically recipients and relays associated to them. 
The main goal is to whitelist mail coming back from those senders and 
relays early in the postfix restriction chain, so it doesn't get hit 
with any client UCE checks you are using (RBL, helo, PTR, greylisting etc). 

WWW: http://mailfud.org/postpals/