Don't know how much this will help you, but this has been our method of migrating new clients, without them losing any e-mail:
First, create a dummy domain, and the real domain on the new server. Put the dummy domain as an alias on each account. For example:
Users:
john.doe@example.com jane.doe@example.com
Aliases:
john.doe@dummy.example.com jane.doe@dummy.example.com
Make sure you set up MX records on the dummy account to point to your new Zimbra server.
Next, migrate all of the data. Whether it be imapsync, PST import, or
however else you want to get the e-mail into Zimbra.
Then go into the current e-mail server, and set each account to forward to the dummy address. So
john.doe@example.com will forward a copy of the email to
john.doe@dummy.example.com, etc.
Then switch the users over to your new system. Then kill the old system (switch DNS, etc). By the time you cut ties with MS, everyone will already be working on the new server.
After everything is said and done, delete the dummy domain, and any aliases.
Caveats: While you are migrating, E-mail will come over from the old server to the new, but not vice versa. So if you move over half the employees, they won't be able to e-mail anyone still on the old system.