This sounds like a tough way to explain that you are interested in using a resource forest in your Exchange implementation.
Here is a stick figure diagram to explain how you could do this, and create accounts in the resource forest with the associated external account of the primary forest.
That scenario isn't completely untrusted because you need to assign the account forest "real users" to the Microsoft Exchange forest mailboxes. This is used in the outsourcing and ASP model where you have an Exchange forest housing the application, and another forest with the account information.
To go further on this, how do you move the mailboxes? Is ExMerge the only option? I believe the Exchange Server tools that have "move mailbox" functionality (Exchange System Manager and Active Directory Users and Computers snap-ins) only allow for movement within one Global Address List (GAL). Is that correct?
Microsoft provides more information on Exchange migration tools -- what they can do and what they can't do -- in this article.
Peter terSteeg, Deployment and Migration Expert
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