[djur]

It seems like a big part of the motivation is that every other entity they have to interact with is using Windows. That's a pretty compelling reason to switch back, and also a pretty good example of how Microsoft is excellent at causing vendor lock-in.

Also, there's not really anything available in the open source world that provides precisely the features that corporate sysadmins look for in Exchange (and Sharepoint, Active Directory, etc.) in an in-house system. I didn't enjoy switching to Exchange from Google Apps post-acquisition, but I don't know of any self-hosted alternatives that provide vaguely integrated email and calendar that don't suck equally as hard as Exchange.