Reconnect previous mailbox after migration

Question

Hello!

We have a exchange hybrid environment with EX2016. I'm wondering how following case is handled:

A user got a usermailbox onprem and, because of misconfiguration, a usermailbox in exo. When you consider the onprem-mailbox as the correct one, the correct way would be to disable the exo-license, migrate the onprem-mailbox to exo and assign the license back again.

The other way around would be to disable the onprem-mailbox and enable the remote-mailbox, depending which mailbox should "survive".

Now the question: In the first case, you disable the exchange-online mailbox and migrate the onprem. Is in this case a "new" mailbox created which a new guid in exo or is the old, cloud-only, mailbox overriden with the data from onprem? Is it possible to recover the data previously stored in the old mailbox or is it gone?

Thank you for your advice!

Chris

Answer 1

Hi @Christopher Stainczyk ,

Now the question: In the first case, you disable the exchange-online mailbox and migrate the onprem. Is in this case a "new" mailbox created which a new guid in exo or is the old, cloud-only, mailbox overriden with the data from onprem? Is it possible to recover the data previously stored in the old mailbox or is it gone?

The data in the old mailbox is gone in this case.

If you choose to disable the exchange online mailbox and then migrate the onprem, you would need to clean up the Exchange Online objects in advance to proceed, so all the content in the mailbox would be deleted as well. See Permanently Clear Previous Mailbox Info.

If you insist using this method to deal with the 2-mailboxes situation stated at the beginning, I'd suggest exporting the data from the old cloud mailbox to PST file up front, and then import it after the migration is completed.

However, the recommended method is the second case as you mentioned above, that is to disable the on-prem mailbox and keep the Exchange Online one. The contents of the disconnected on-prem mailbox could be restored to the Exchange Online mailbox. For detailed instructions, hopefully you can find the link below helpful:
How to recover when a mailbox exists in both Exchange Online and on-premises

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