I have been using Avast Server Edition with the Exchange Server plugin for several months now. It works beautifully.
I have the Exchange Server plugin set to scan messages at the transport stage (a feature available only if you use Exchange Server 2003). It routinely catches viruses in incoming messages that other products have missed. There has been no problem with Exchange database corruption.
The Avast Small Business Server suite includes the Excahnge Server plugin. It's the least expensive way to purchase this combination.
Ken