Some spam filter software from a recent issue of "Maximum PC" magazine:
Spam Gourmet weblink
POPfile weblink
And there's an open-source tool called "Tagged Message Delivery Agent" that can be installed on mail servers and acts as a blacklist/whitelist. One of the functions it can do is hold the first message from a given e-ddress, send a message back to that e-ddress requesting a reply to confirm identity. If/when such a reply is received, that e-ddress is added to the "whitelist". URL for TMDA is:
Tagged Message Delivery Agent weblink