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.