It is possible to create a Linked Mailbox in Exchange 2007 in which the resources and the user accounts are part of a resource forest; the resources reside in one forest while the user accounts are located in another. Setting up a Linked Mailbox is similar to the process of creating a normal one in Exchange Server 2007. From within the Exchange Management Console, fire up the “New Mailbox…” wizard by clicking on the appropriate action with the Recipient Configuration work center selected. Instead of choosing User Mailbox however, skip to the bottom and select Linked Mailbox instead.
In the next step, click on “Existing user” and “Browse…” to the user from another forest. Click “OK” and “Next.” Moving ahead, determine the location of the mailbox database the same way as you would for a standard user account in Exchange 2007 before clicking on “Next.” On the Master Account screen, information pertaining to the trusted forest or domain is required, which is selected by clicking on the “Browse…” button.
In order to access the domain using a different username and password, simply select the “Use the following Windows user account to access linked domain controller” checkbox. Key the appropriate information into the text boxes. Click “Next” and follow it through to completion to create a linked mailbox.