During some of my engagements, I have come upon an issue where customers running an Exchange Hybrid deployment cannot move mailboxes 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:

  1. Using Windows PowerShell, connect to the Exchange Online organisation and run:
    Get-Mailbox <MailboxName> | Format-List ExchangeGUID
    Then note the ExchangeGUID.
  2. Using Exchange Management Shell on the on-premises Exchange organisation and using the ExchangeGUID from the previous step, run:
    Set-RemoteMailbox <MailboxName> -ExchangeGUID <GUID>
  3. Wait for directory synchronisation or force a manual directory synchronisation.

Once the above steps have been completed, retry the migration for the affected mailbox!