Exchange 2010/2013 coexistance mailflow issues: 421 4.4.2 socket error.

So I am in Exchange 2010 SP3 / Exchange 2013 SP1 co-existence. 

I can send from a test 2013 user to external and 2010 users internally on the domain. But I cannot send to the 2013 test user. I get:

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451 4.4.0 Primary target IP address responded with: "421 4.4.2 Connection dropped due to SocketError." Attempted to failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

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I've been looking at this for example: 

http://support.microsoft.com/kb/979175

But no matter where on the 2013 receive connectors I add Exchange Server Authentication, it still doesnt work.

Theres so much stuff on this error message, but everyhing I find seems to be 2003 / 2010 or other coexistance which is different to my environment.