I can't get mail flow from Exchange 2013 to my 2007 server working. I'm pretty sure I know the issue but let me give you an idea of the environment:
Old Server
One Exchange 2007 RU10
NETBIOS is "MAIL"
External URL was mail.domain.com
New Setup
2 CAS, 3 MBX servers
New Exchange 2013 is now the mail.domain.com both for internal and external (internal and external dns the same)
Old server NETBIOS is still "MAIL" but I created A host record for "LEGACY"
NEw server is now the mail.domain.com both internal and external
What I noticed: If I put static dns in hosts file on the mailbox servers then mail delivers. My guess is Exchange 2013 is trying to forward messages to 2007 server using the DNS name "MAIL" or "MAIL.domain.com". Since MAIL now points to the Exchange 2013 load balancer it creates a loop.
I would do a "Test-Mailflow" command and it would fail. After putting in hosts file record the "Test-Mailflow" works to 2007.
But what problems am I going to come out of putting the hosts file record in? If I will have problems how do I resolve this? I can't change the server name of the old server can I since Exchange is already installed?