I have a Windows Server 2012 Server with Exchange 2013 CU2, Revision 2.
I can send mail from Exchange 2013 to users on Exchange 2013, I can send mail from Exchange 2013 to people on the Internet.
I can send mail from Exchange 2007 to users on Exchange 2007, I can send mail from Exchange 2007 to users on Exchange 2013, I can send mail from Exchange 2007 to the Internet.
I cannot send mail from Exchange 2013 users to Exchange 2007 users. The message is stuck in SMTP Relay to Mailbox Delivery Group, and it receives a 421 4.4.2 Connection dropped due to socket error.
I have verified that there are MX records for each server in the domain. The two servers are next to one another and connected to the same Gigabit switch.
Looking at the receive connector logs it gets to passing the allowed forms of Authentication, the Exchange 2013 server sends back X-ANONYMOUSTLS - the Exchange 2007 server passes the certificate, last line shows - LOCAL and that is all. I can see 3 attempts in the log during each retry to send the mail from Exchange 2013.
I have tried to enable Verbose logging on the Exchange 2013 Send connector, and have not been able to see a log created for this delivery from Exchange 2013 to Exchange 2007.
Exchange 2013, has been upgraded to CU1, CU2, and CU2 Revision 2. Exchange 2007 has the latest SP and updates.
I created a self signed certificate and assigned that to SMTP on Exchange 2013 in case it did not like one of the names of that server being missing from the Certificate created during install. That did not help.