Exchange 2010 - 2013 coexistence OWA and activesync issue

Exchange 2010 - 2013 coexistence with one mailbox moved to 2013.  We can send and receive using the 2013 mailbox internally as both servers are talking nicely to each other.  When trying to set up iphone to 2013, activesync errors on the 2010 server below:

The Exchange ActiveSync user Test has a mailbox on a Client Access server running a newer version of Exchange. Exchange ActiveSync doesn't support proxying users to Client Access servers running a newer version of Exchange. The user needs to connect to a newer Client Access server.

I am sure it is a simple solution.  It is important I am able to have both activesync working on both servers while migrating over 200 mailboxes as most users are setup with smart phones and cannot be without email for too long.
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Simon Butler (Sembee)ConsultantCommented:
With OWA, if you have an external URL set, then it needs to be different to the one set in Exchange 2013, as Exchange will see that as a request to redirect rather than proxy.
If you remove the external URL then it will proxy the request to the Exchange 2010 mailbox.

For anyone else reading - that does not apply to Exchange 2007, which can only redirect.

Simon Butler (Sembee)ConsultantCommented:
The error is pretty self explanatory - you need to direct external ActiveSync traffic at the new server and let Exchange proxy to the old server.
On the old server you will need to remove the external URL so that Exchange knows it needs to proxy.

accucomAuthor Commented:
so just remove the external URL from Server Config - Client Access - Exchange ActiveSync?

Is this the same for OWA?  Will exchange 2010 auto redirect to
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