Another great feature in the Exchange 2010 Management Console is that you now can do cross-forest mailbox moves using the new “New Move Request” wizard. To launch this wizard right-click a User Mailbox in the EMC, then select New Move Request in the context menu as shown in the figure below.
This brings up the wizard shown next. Here you can specify to which Exchange organization you want to move a mailbox.
Note
Before you can perform a cross-forest move, you must add the Exchange org in the target forest to the EMC. In addition, you must have the AD account of the source user mailbox migrated/replicated to the target forest using ILM or a similar tool. Yes doesn’t work like the Move-Mailbox cmdlet did in Exchange 2007, where the AD object would be created if it didn’t already exist.
– Henrik Walther