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I have two forests, forest A, forest B.

Forest A has no Exchange server. Forest B has 2010 Enterprise.

Forest A does not trust Forest B. Forest B does trust Forest A.

The users are in Forest A, and I want Forest B to use those users in A and give them mailboxes in the B Exchange server but authenticate with A's domain controller.

Is this possible? If so how can I do it?

I cannot use Federation Services because I only have one 2010 Exchange server and it is in Forest B.


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These forests. Are they both local? Will they / are they seperated geographically? Tell me a little more about the use case. – JohnThePro Mar 6 '12 at 18:27

Obviously, without the caveat information (which I'll happily update this answer with) my gut instinct is to say you need to build a 'resource forest'. Effectively, what this does is it allows users from one Forest access Exchange resources within another forest. This can be used effectively when two companies merge but have no desire to restructure their AD forests. This TechNet article will walk you through setting up Exchange in this fashion.

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