Small network. One Exchange 2010 server. I just added Exchange 2013 on another VM with the intent of migrating everyone from 2010 to 2013 (was on 2003 as of about six weeks ago, so this is real progress, believe it or not!)
Mail sent from outside does not get to the test mailbox on the 2013 server. Eventually, that 2013 server will be setting behind a NAT to port 25 on the router, but in the interim, I need mail to flow through Exchange 2010 and be delivered to 2013.
I am sure, or at least highly suspect, that there is something simple and obvious that I missed in Mail Flow that will allow messages hitting the Exchange 2010 server to be moved over to the user's mailbox that is in a DB on the Exchange 2013 server.
For what it is worth, I have mail delivery configured on one mailbox that forwards mail sent to the domain admin account out to my external e-mail address (in addition to, not instead of, putting a copy in the admin mailbox). This works, even when the message never actually shows up in that admin mailbox. I am sure that is significant and probably a big hint also, but just cannot see it at almost 3:00 am in the middle of this migration.
I will need 2010 & 2013 to coexist for maybe only three days while I get everything moved over.