I use this, and it's excellent.

http://www.mailwasher.net/

Plus, it's free (which is nice)

Check out the features on it, and I am sure you will agree, that is very effective.

I'm looking at mDaemon as a mail server at home ($100 for 6 users).

It has an option to check for spam as it arrives (it checks the senders address/IP against a couple of lists of known spammers (one of them is ORDB.org), and if they match, mDaemon returns a "User Unknown" message, which increases the chances of being removed from that spam list.)

The only problem I've found so far is that sometimes the spam lookup fails (I've yet to investigate whether that's to do with the website it uses, or mDaemon itself), so there may be one day every couple of months where all the spam gets through - but that's only a minor quibble!

The only comment I would have about the Postini product is how good are it's heuristics - what proportion of false positives does it produce?

Just my tuppence worth!

Rgds
AndyF