I have installed Exchange 2013 in the Exchange 2010 environment and created some test users and didn't notice any issues.
After initial testing I have moved some of the production mailboxes from Exchange 2010 to Exchange 2013, intermittently i see that emails from Exchange 2010 users for Exchange 2013 users are stuck on Exchange 2010 HUB servers in a Unreachable Queue.
Initially it was showing DNS error , SMTP SEND error in the Last Error, but now it doesn't show anything and the emails sit in Unreachable Domain.
I checked different DNS options etc, it only works when I bounce the AD Topology Service on the HUB Server.
After bouncing the service if i check, then i can see the "Hub Version 15" queue showing and the emails are delivered to the Exchange 2013 mailboxes.
Not sure what is causing the problem as I checked both the Exchange 2010 and Exchange 2013 servers are in the same subnet and also they are in Same AD Site.
Can anyone helpme with this, i cannot proceed with the migration before I resolve this issue.
mdimthyas