The exchange management console defaults to the currently logged in domain. If you’ve got multiple Active Directory domain trees in your forest this poses problems when managing mailboxes from the other domain trees. To view all of the domains in the exchange console you need to select the Recipient Configuration container, and from the actions scope select “Modify Recipient Scope…”.

Bear in mind that in large environments you may not wish to view the entire forest at once, however using the same process you can change your scope to the domain wish to administer within exchange.