⟩ Under Exchange 5.5 I could not restore a single mailbox without 3rd party products. With Exchange 2003, is it any easier to restore a single mailbox or back up a single mailbox?
Yes and no. Under Exchange 2003, a mailbox is not deleted immediately when a Windows account is deleted. Although restores have been greatly improved with the new Recovery Storage Group (RSG) and the Volume Shadow Copy Service, there is no built in mechanism for backing up a single Exchange mailbox. This would still require a 3rd party brick level backup utility.