I created my first test mailbox on exchange 2013, when I mail a user still on 2010 in our org it just goes to queue, in fact there is a bunch of stuff in there.
The test account can however receive email from 2010 just fine.
And whats really strange is that it works perfectly with external accounts both outbound and inbound so its some sort of internal routing from 2013 to 2010.
All I see is 400.4.4.7 Message delayed in the queue
Ideas?