During some of my engagements, I have come upon an issue where customers running an Exchange Hybrid deployment cannot move mailbox created in Exchange Online back to the on-premises Exchange organisation.
This is usually due to an ExchangeGUID not being stamped onto the on-premises Exchange mailbox at the time it is created.
If the ExchangeGUID is not present the Exchange on-premises mailbox, the mailbox cannot be moved to the on-premises Exchange organisation.
To resolve this is fairly easy. You need to find the ExchangeGUID of the Exchange Online mailbox and manually place this onto the on-premises Exchange mailbox prior to migration. To do this follow the below steps:
- Using Windows PowerShell, connect to the Exchange Online organisation and run:
Get-Mailbox <MailboxName> | Format-List ExchangeGUID
Then note the ExchangeGUID.
- Using Exchange Management Shell on the on-premises Exchange organisation and using the ExchangeGUID from the previous step, run:
Set-RemoteMailbox <MailboxName> -ExchangeGUID <GUID>
- Wait for directory synchronisation or force a manual directory synchronisation.
Once the above steps have been completed, retry the migration for the affected mailbox!