Not sure why you would do this.... but everyone has their reasons...
Probably not recommended at all. Unity needs to communicate with the Exchange server for not only MWI, but you also have AD Domain connectivity, SMTP delivery (drop, fetch) DNS resolutions, authentication, etc.
So quite a few ports you would have to open up, then cross your fingers it stays. If you lose connectivity with Exchange, your MWI will go out of sync and users will complain.