I've run into a snag implementing an on-premesis Exchange 2013 Server with an Exchange 2007 Coexistence. I've migrated one mailbox to the EX2013 server for testing and can send mail from that account Outbound, but have no Inbound Mail Flow.
I cannot Telnet to ports 25 or 587 on the Exchange 2013 server, but I can Telnet port 2525. I just get the blinking cursor on failed Telnet attempts and return to a prompt when I hit a key. I can, of course, Telnet to 25 and 587 on the existing Ex2007 server. The Exchange 2007 server is intended to remain as the Transport server for the near future, with just mailbox and CAS roles on the Ex2013.
The other primary clue I have is the delayed delivery messages I see for messages sent to the mailbox on Ex2013. On the Ex2007 server, I have a hub version 15 queue and the Delivery Failure message is 451 440 Primary Target IP address responded with "421 4.4.2 Connection dropped." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts or delivery failed to all alternate hosts.
Lastly, I can tell you both Exchange Servers are on the same, internal subnet. Its a single site and all servers are behind a Sonicwall firewall - which shouldn't be in the way anyway. I've tried turning off Windows Firewall on both and neither has any AV program installed.