Exchange 2013 / 2010 Coexistance - ActiveSync

Hi Guys.

Hopefully someone can advise me.

I have the following setup

1 x Exchange 2010 All Roles

1 x Exchange 2013 Mailbox Role

1 x Exchange 2013 CAS

I have changed our firewall so that Port 25 and 443 go to the 2013 CAS. Users internally and Externally can use Outlook fat client, iPhone Active Sync and OWA with no issues.

If i move an account over to Exchange 2013 then Active Sync fails to connect. OWA is fine and Outlook fat client has no issues.

Any ideas on what i need to change so that a user that is moved from 2010 to 2013 can use Active Sync?

Thanks in advance