Migrate and properly store Exchange on-premise inactive mailboxes to Office 365

Curious_Kevin16  

Hey,
Hmm good question.
I’m not quite sure but i think you can migrate without an issue even accounts which doesn’t hold a license. If you want/need to use it ( logon to that accounts ) you need a valid license especially if you hit some quota limits.
The question is what do you want to do with that mailboxes. Do you only want them migrated for compliance requests or will they be accessed by real people on regular basis ?

If you need them only for compliance reasons I would change them into inactive mailboxes. For that you need a retention policy which hold your data after deletion. If this is in place you can delete the associated AAD account and the mailbox will become an inactive mailbox which can be „accessed“ via the compliance center.
One thing to consider are auto expanded archives because it is a bit of a „problem“

https://learn.microsoft.com/en-us/microsoft-365/compliance/inactive-mailboxes-in-office-365?view=o365-worldwide#inactive-mailboxes-and-auto-expanding-archives

Not sure how many big mailboxes you need to migrate. But the following should work without any problems:

Create hold/retention policy.
Assign a valid license and do the migration.
Delete the migrated AAD account this will release the license and „convert“ the the mailbox to an inactive one.
You could do that step by step deepening how short on licenses you are…
Hope this helps

Cheers

Chris