For Microsoft Intune or Configuration Manager to directly manage mobile devices, users must enroll devices with Intune. For mobile devices that users haven't enrolled, you can enable Exchange ActiveSync (EAS) management using the Exchange connector. Devices can be managed with either on-premises Exchange servers and cloud-hosted Exchange on Microsoft Office 365.
If you find yourself in a situation where built-in Exchange connector in Configuration Manager or On-premises Exchange connector tool in Intune standalone environment does not discover devices from on-premises Exchange, please continue reading.
This could happen after you performed a mailbox migration. Both built-in Exchange connector in Configuration Manager console and on-premises Exchange connector in Standalone Intune may fail to discover devices if ServerName attribute on the mailbox does not reflect the correct mailbox server name. You can view current value of ServerName attribute by running Get-CASMailbox against a few accounts. When this happens, no errors are logged in log files but the end result would be no devices are discovered. This may effect either all or a few users.
The best way to fix this issue is to update the value in ServerName attribute to reflect the correct mailbox server name. Fortunately, a colleague in Exchange team published an excellent blog post covering the details. You can find it here.
If you are in middle of a migration i.e. from old version of Exchange to a newer release and ServerName attribute displays the correct mailbox server name but devices are still not discovered, please configure Exchange connector in SCCM to point to a specific CAS server instead in your new Exchange environment.
Exchange Connector - Properties - General - On-premises Exchange Server - tick Specify Exchange Client Access Server and type the FQDN along with Active Directory site name