Hello,
We cut-over to a new 2013 CAS server (external) by changing the NAT from the old 2010 CAS. ActiveSync proxy to the old 2010 CAS/Mailbox server (internal) was working well with one exception. One iPhone showed no error on the phone, but would not load any new mail. This was also the only device we tested that had been connecting to the old 2010 CAS server before the cut-over, so there were no changes to the mail profile on the phone. Rebooting the iPhone did not resolve the issue.
In the IIS logs of the 2010 CAS/Mailbox server shows the following error for each ActiveSync proxy request from the device:
2013-11-18 23:07:58 192.168.126.3 POST /Microsoft-Server-ActiveSync/Proxy/default.eas User=jsmith&DeviceId=ApplC38JK0TJDTTN&DeviceType=iPhone&Cmd=Sync&Log=PrxFrom:172.16.16.4_V141_Cpo20000_Fet20015_Error:MissingCscCacheEntry_ 443 DOMAIN\OWA-2013$ 172.16.16.4 Apple-iPhone5C1/1101.501 441 0 0 20031
We backed out all the changes, and then configured the new 2013 CAS server to serve ActiveSync on a different external URL for testing. When the same device was configured to use this URL, the proxy succeeded.
I'm happy to provide any other relevant details.
Thanks,
Travis