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.   

    + CategoryInfo          : ObjectNotFound: (:) [Test-ActiveSyncConnectivity], LocalizedException
    + 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

• Edited by Mark Schuljak 9 hours 21 minutes ago more info

Hi Mark,

Thank you for your question.

By this error, Exchange CAS server name could not find in AD. Firstly, we should make sure the name of CAS server is correct by the following command:

Get-ExchangeServer

If it is correct, then we could navigate to the following path to make sure the CAS server is existed by ADSiedit.msc:

Configuration [<domainController>.contoso.com]> CN=Configuration,DC=contoso,DC=com> CN=Services> CN=Microsoft Exchange> CN=<OrganizationName> CN=Administrative Group> CN=Exchange Administrative Group>CN=Server

Then we could right click Property of Exchange server and check the following item:

1.        Cn
2.        DistinguishedName
3.        Name

We could use the following command to check if the issue persist:

Test-ActiveSyncConnectivity ClientAccessServer esri\red-inf-mxcs-p2

Notice: this command is based on the correct Exchange CAS name.

If there are any questions regarding this issue, please be free to let me know.

Best Regard,

Jim