"SpamTitan is fantastic. We have it running at Expression College and it's been great. Super low system requirements, we had it running on a Dell desktop for 6 months and it processed 2.5 million emails without any issues (except when the hard drive died, but that wasn't their fault). We are moving over to their hosted platform now and so far it's really straight forward.
I've used other systems, mostly free open source stuff, and this is so much easier to set up and maintain for me.
Their new release, 6.0, was a painless upgrade that the system handled by itself. Don't really have any complaints about the system. The only thing I would like to see is a version 2 (ideally) HyperV as well as the native install and VMware option."