If you're running a mail server for your company on OS X do yourself a big favor and check this out! I run CommuniGate with MPP and could not be happier. I use MailShell for anti-spam and Sophos for AV. These two really cut down on the junk.

If you don't want to pay for Sophos and MailShell then you can plugin open source for filtering. I used ClamAV and SpamAssassin for a long time before I needed more options. (ClamAV can not clean a virus so mail with legitimate attachments gets rejected. With Sophos I can clean the virus from the message -- usually Word macro viruses -- and still deliver the mail.)

The support is also top notch. I swear these guys never sleep.