A friend came across this issue while migrating from Exchange 2010 to 2013. Users were unable to receive/send email from their mobile device after migration. This continues for up to 4 hours or so then starts working out of the blue.
It’s actually a bug with certain versions of Exchange 2013.
To fix, simply recycle the following App Pools in IIS on the CAS server:
A fix is in place for CU5, so if you are on that version or higher this fix “shouldn’t” apply…….