We have installed Exchange 2013 CU6 working in co-existence with Exchange 2007 SP3 RU14. Due to some issues with EAS we have only migrated a few mailboxes to 2013 so far.
We have spotted that when a 2007 mailbox user sends a 2013 mailbox user an email there is a delay in the 2013 recipient receiving the message. In 2007 queue viewer it reports:
Hub Version 15 451 4.4.0 DNS Query failed. The error was: SMTPSEND.DNS.NonExistentDo
main; nonexistent domain
The messages will get through eventually but between 15-60 delay as it is retried.
I have followed this article but the ExternalDNSServers and InternalDNSAdapterGUID settings are correct as well as nslookup of the 2013 server name
Email flows normally between 2013 to 2007 and between 2013 homed mailboxes just not where the sender's mailbox is on 2007 and the recipient is on 2013.
During the deployment we followed the MS Exchange Deployment advisor and disabled IPV6 on the Exchange 2007 server in the registry.
Can you help smooth over the transition period?