Try GFI MailEssentials here

It'll give you a 30/60/90 day trial, (I forget which), and it will give you breathing room. If you like it I found it to be quite cost effective.

Be careful of false positives to start with. If you have a lot of mail it quickly backs up and whitelisting all the non-spam domains becomes a problem. Once the initial phase is over it's pretty much a "fire-and-forget" thing.