We have a co-exist Exchange 2010 / 2013 environment and we are seeing this in Event logs on our our two CAS only. These are in production but all data is still in 2010, 2013 just the proxy right now:
Test-ActiveSyncConnectivity-ClientAccessServer "RED-INF-MXCS-P2.esri.com"
esri.com/Foglight/fgl_wmi
Local-ConsoleHost-Unknown
5996 powershell.exe
The specified Client Access server was not found in Active Directory.
+ FullyQualifiedErrorId : [Server=RED-INF-MXMB-P3,RequestId=a2c66548-3d38-4426-9314-7ff807213875,TimeStamp=5/18/20
15 9:33:49 PM] [FailureCategory=Cmdlet-LocalizedException] 18DA4E92,Microsoft.Exchange.Monitoring.TestMobileSyncCo
nnectivity
+ PSComputerName : red-inf-mxcs-p2.esri.com