Has anybody got any guides or recommendations as to how to progressively replace our mail server with Zimbra?

I had initially planned to use one of the more obscure domains we have which does not get used (ie: our-domain.net) but I would have to do most of the testing manually.

The other approach would be to have zimbra handle user1, user2 and then forward all other email to the current server. This way I should be able to add users once I feel we have things tightly wrapped.

Is this something that can be done and any pointers as to how?

Many thanks