CustomerCentrix

We are currently using SpamBayes and Thunderbird native spam protection. SpamBayes seems to work a little better, but does not integrate with Thunderbird. Supposedly, Thunderbird uses the same algorithm as SpamBayes, so it may just be an incorrect perspective of what works better because you have a bit more information and control using SpamBayes. My goal is to improve how this works somewhat by instituting some server level prevention as well. There are many options, including a couple that come integrated with Plesk:

SpamAssassin
  Spam Assassin
  magma.com config

MAPS lists
  crossbar.net
  sorbs.net
  dedicatedcentral.com (Plesk)

General info that applies to Qmail and anti-spam
  Chris Hardie
  Life with Qmail
  Qmail - antispam

The benefit of stopping spam at the server level is that clients don't have to see it at all. This is also the drawback because of the "false positives" where email is marked as spam when it should not be. If you take this to the extreme and use one of these tools to reject or delete mail, then you will occasionally lose something that you really wanted to receive. SpamAssassin is typically used to just flag the subject line of email and then pass it on to clients. To me, this seems inadequate and somewhat redundant if you are going to use something at the client level anyhow. I would like to take this one step further, and actually move this email to another mailbox where it would be stored and recovered if necessary. I would also like to be able to keep statistics on volume and percent categorized as spam to get a feel for whether it is working.

Also, some good information available here:
  Chris Linfoot