Outlook 2013 client not sending internal emails from mailbox on Exchange 2010 to mailboxes on 2003

We are in the process of upgrading our exchange server to 2010 from 2003. I have a couple of users with mailboxes on the new exchange 2010 server who are having a very odd issue. Everything works but their outbound internal email to mailboxes

on the exchange 2003 server. They get no bounce back messages, but they do not go through when sent from their Outlook 2013 clients. If sent from their mobile devices or the OWA site, they go through no problem. They send/receive under all

other circumstances fine. I see nothing in logs, this is only thing I have seen in their synch issues log in the clients (I replaced server name and user name to protect the innocent :-) ):

8:50:54 Synchronizer Version 15.0.4420

8:50:54 Synchronizing Mailbox 'Doe, John'

8:50:54 Synchronizing local changes in folder 'Drafts'

8:50:54 Uploading to server 'IM06.abcdefg.abcdefg'

8:50:54 Synchronization of some deletions failed.

8:50:54 [0-130]

8:50:54 1 item(s) deleted in online folder

8:50:54 Done

Please help!!!!

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