We upgraded from Exchange 2003 to Exchange 2010. All users and mailboxes were migrated, but 1 mailbox needed to be recreated, as it never sync'd. The mailbox was recreated but the AD user was not recreated. The issue we are having is that when staff try to send an e-mail to this mailbox (by selecting it in their global contact list, not from cached) they get a bounce back. I believe they are getting this bounce back because the global contact for this mailbox is pointing to the old mailbox. The mailbox can still receive email if you type in the full address, but not if you select the global contact.