Why? What was wrong with Zimbra?
We were running Exchange in house, but were considering moving to Zimbra. We ended up migrating to hosted Exchange (from Microsoft). I've been completely un-impressed by hosted Exchange.
Every time I 'try' to use that god-awful web interface (most of the time I just get some power shell error), I think we made the wrong choice. I would like some feedback as to why you went back to Exchange?
My long term goal has been to get Microsoft out of our data center. Especially with the shakeup going on at Microsoft. Getting Microsoft off the desktop is not very realistic for us yet. My users would revolt if I took away their MS Office. I made a push a while back for Open Office, even going so far as installing for some users, but there are too many problems for us to go there (we use a lot of VBA macros - yes, I know!).