We have 60 users and around 30 mobile devices. The cost of hosting costs a lot. I wanted to redo the email server from the start but waited too long obviously. The annual cost of hosting is like 3 to 4 times the cost of having it in house. I will confer with the brass though.
For that many users (which isn't that many) I'd probably consider hosting. I know it's the standard IT thinking to be in control of everything end-to-end, but some things are worth letting go, IMHO.
If you had a couple hundred users, I'd probably keep it internal.