I am in the middle of migrating a client from Exchange 2007 to Exchange 2013 and it's gone fine up to this point. I've created a new test mailbox on the 2013 server and it can send to the Exchange 2007 server and externally fine but it does not receive messages from the 2007 server based users.
Currently the messages are sitting in a queue on the Exchange 2007 server awaiting retry but there are no error messages on the queue. I can telnet to the 2013 server on port 25 and test messages I send directly over SMTP work fine and are delivered, it is only messages sent from the 2007 server to the 2013 server that have this issue.
Would have to say my Exchange server is limited and this is the first on premise migration I've done in ages it's all been to Office 365 via Skykick in recent years so any assistance gratefully received