I thouht I saw someone mention greylisting on their server. How did you acomplish this with Plesk? The greylisting mods I've seen for qmail require patching the source and recompiling which I imagine would make Plesk very unhappy.

I had a problem with it blocking legit email.

for example yahoo.com would take over 3 hours to resend, thus it would never go through.

I used the RPMs from http://www.atomicrocketturtle.com/atomic/psa/9/RPMS/