When users open Outlook on their local machines it will first find the SCP in the Active Directory and does not use AutoDiscover. That means that users will be logged onto the old (decommissioned) server, while external AutoDiscover outside of the clients network works perfectly.
What the Partner/Customer will need to be able to complete the steps, for example
- Admin access to the old server.
Using PowerShell on the old server, you can run the following cmdlet in the Exchange Powershell Module.
Remember to only run this if all local exchange servers have been decommissioned.
Get-ClientAccessServer | Set-ClientAccessServer -AutoDiscoverServiceInternalUri $null
If you want a temporary fix (for testing), you can disable the IIS services on the CAS server, preventing logons, Outlook will automatically use Autodiscover when it finds it cannot log into the local server.