Abstract: You setup a additional Exchange 2016 for Co-Existence with Exchange 2010. You switched the name space and then recognize that the internal Exchange 2010 users are unable to connect to Exchange while new created Exchange 2016 users could connect fine with there Outlook clients.

The full error messages the Exchange 2010 users got are:

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Microsoft Outlook
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The name cannot be resolved.  The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action.
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OK  
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Microsoft Outlook
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The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action.
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OK  
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However the access from external is working (Outlook WebAccedd [OWA], AutoDiscovery, Outlook Anywhere [OAW]). Also internal OWA is working. So something must be wrong with the internal Outlook connection.

 

Troubleshooting:

1.) Check the name from the internal access point for Outlook (Outlook Anywhere as we use Exchange 2016, there is no MAPI access point any longer!) This can be done via:

Get-OutlookAnywhere -Identity "exch2016-01\Rpc (Default Web site)" | select InternalHostname

2.) Check the name from the CAS Array on your Exchange 2010 this can be done via:

Get-ClientAccessArray | Select Fqdn

additional check which RpcClientAccessServer is set on the Exchange 2010 MailBoxDatabases via

Get-MailboxDatabase | Select Name, RPCClientAccessServer

 

If both names (RPC Client Access Array/Server and the internal Outlook Anywhere) names are the same, you will cause a routing loop once you switched over the default name space to Exchange 2016. So the solution is to fix at first the name issue.

 

Solution:

A possible solution is mentioned here and the solution discussed there is to change the RPCClientAccessServer to a single Exchange CAS Server (e.g. hostname) via:

Set-MailboxDatabase –Identity "<Database Name>" –RPCClientAccessServer "exch2010-01.contoso.local"

Keep noted that your 3rd party server (e.g. AirWatch, BlackBerry, ...) and as well your users needs to have access to this server!

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