I used to just rely on desktop based spam filters, but it was too much work to update all the computers on the network when new versions came out. So I signed up for a server based filter with Sentinare Spam Filtering. Greater then 99.6% is awesome and all server side, so nothing to download and install and maintain. I was gonna use Postini but I heard they are even worse then iMail at 85% effectivness and they have a $250 monthly minimum, so Sentinare was a way better deal at less the $3 a mailbox. I have had zero false positives thus far and really happy with the speed, service & support.