If i install the Hub transport and client access server roles at domain A then i install the mailbox server role at domain B. Will the mailbox enable users reside at domain B able to make use of the HT and CAS at domain A to access their mailbox? All under the same Exchange organization and the HT, CAS and M are all under the same site. P/S In the mid of migrating Exchange 2003 to Exchange 2010, goin to create a new domain to hav all the exchange server 2010 roles fall under this domain...at the same time during the transition, need to ensure the CAS able to proxy back the request to Exchange 2003 back end should the mailbox still reside in there. Please advise..thank you.

Assuming you're talking about all 2010 roles (not a mix of 2010 and 2003), I would expect this configuration to work, however I’ve never tested the exact scenario. Exchange is a forest-wide technology and doesn’t really care about AD domains (provided you run the necessary domain preperations). The entire configuration is stored in the Configuration Naming Context which is replicated to every DC in the forest. Having said this, you want to be moving to a 1 forest / 1 domain model. I do not see any need to create the additional complexity of additional domains in the forest. What is the supporting logic here? Mike Crowley Check out My Blog!