I mentioned before that ASSP (Anti-Spam SMTP Proxy Server) is a great spam prevention solution. The install was a little complicated and the configuration took awhile to get just right. However, in the long run, I found out it was not for me. Even when I finally had it tweaked right it was blocking a few valid emails. So I switched to MagicSpam which is a commercial extension for Plesk. I disabled SpamAssassin as well. MagicSpam doesn’t have the ability to judge emails by subject or message, but it still blocks a lot. You can whitelist or blacklist IPs or emails addresses (and with the use of wildcards you should be able to block entire domains). It’s light on resources and should result in no false positives to be blocked.