We just installed a new Exchange 2013 server and have coexistence with an existing Exchange 2010. Both are multi-role Mailbox/CAS. We are in the process of migrating completely to the 2013 server.
With user mailboxes residing on the 2010 server's database inbound and outbound mailflow is functioning properly to internal and external recipients, and is being proxied through the 2013 server.
When we move a mailbox to the 2013 server's database we are able to receive mail from internal and external users, but email sent from this mailbox is not received by external addresses.
There is only one send connector, and both the 2010 and 2013 servers are listed as source servers.
Delivery report shows: Submitted, Pending, Transferred, and "handed off to another email system".
There are no messages stuck in the queues on either server.
The test mailbox user does not receive a NDR or other bounceback
If we move the mailbox back to the 2010 database we can once again send mail externally.
What other troubleshooting steps can we take to determine the cause of this issue?