How To Clear msExchMailboxGUID String From Exchange Online.

If you have been running a hybrid environment with your office licenses running in the cloud but you still have your Exchange server on-premise, you might be looking to migrate your mailboxes to Microsoft 365 Exchange Online.
Note: This guide should not be followed if you’re using Hybrid Exchange to migrate mailboxes.

There are plenty of tools that can help you migrate your user’s mailbox data to Exchange Online. One example could be MigrationWiz, but if you have started using ADsync then you might have “msExchMailboxGUID” turned on in ADsync. This causes Exchange Online to think that you have your mailbox is located on-premise, therefore you won’t be able to use the mailbox in the cloud unless the “msExchMailboxGUID” string is clear from the beginning.

Instructions:
* This is not supported if you have performed a synchronization using DirSync.
* This is supported if you perform a synchronization using AAD Sync or AAD Connect.

Start by stopping the automatic ADSyncScheduler by the following command via PowerShell.

Run “Synchronization Rules Editor” as administrator.

Find ”In from AD – User Exchange” and click edit.

Here you choose to edit the original sync rule.

Under “Transformations” find the string called ”msExchMailboxGuid” and change the values to the following:

Expression – msExchMailboxGuid – NULL – Apply Once – Update.

Click “SAVE” and run a full synchronisation.

You can follow the status of the GUID string via the following PowerShell command.

*** REMEMBER TO CHANGE EVERYTHING BACK AND TURN msExchMailboxGuid OFF IN SYNC SETTINGS ***

Finally, you can re-enable the ADSyncScheduler.

If you have any questions / feedback or would like to correct me on any of the stuff above.
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