I recently ran into an issue with the Exchange Management Console (EMC) trying to contact old non-existent domain controllers after a Active Directory domain upgrade. When I tried to access parts of Exchange through the Exchange Management Console, it would prompt a message saying that "Unable to find 'OldDC.domain.local' computer information in domain controller 'OldDC.domain.local' to perform the suitability check. Verify the fully qualified domain name. It was running the command " or "OldDC.domain.local isn't a fully qualified domain name (FQDN). Please provide valid FQDN".
To solve this problem, I just needed to delete the MMC cache for the Exchange Management Console at "C:\Users\%username%\AppData\Roaming\Microsoft\MMC\Exchange Management Console".