Hello, I am in the beginnings of an Exchange 2010 to 2013 migration and I'm running in to an issue with ActiveSync.  When an existing mailbox is still on the 2010 database, ActiveSync is fine.  When I move the mailbox to a 2013 database, ActiveSync stops working.  I get authorization errors on the device that I'm testing with and I get a 403:Forbidden Access error when using https://testconnectivity.microsoft.com.

Also, ActiveSync is the only thing that appears to be broken.  Mail flow to and from an Outlook client is fine after the mailbox is moved.  Everything else seems fine.  My org is very dependent on checking email with mobile devices so I can't move forward until I find the fix for this.

I'm new to this Exchange admin game so I've checked everything I can think of and have gone through numerous forums trying to find the answer to this.

Here is a breakdown of our environment:

2010 - 3 servers (2008R2), 2 have CAS role, 2 have mailbox role

2013 - 2 servers (2012R2), both have CAS/mailbox roles

I've set up a DAG between the two 2013 servers and set up a couple of databases.  Let me know if there is more info that is needed.  I'm stumped.

Is the account a member of any special groups like Domain Admins?

No, it is a generic test account.

I am seeing this error in the Event logs on my 2010 CAS

The Exchange ActiveSync user <domain>/<user> 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 guess that explains why it isn't connecting, I'm just not sure how to fix it.