In any Exchange organization, it is common to add and remove mailbox databases, especially the default ones Exchange creates with the install.
Generally when you want to remove a mailbox database you would need to move any mailboxes, arbitration mailboxes, archive mailboxes, plans, etc. however in Exchange 2016 there is one more.
If you try and remove a mailbox database or perform an uninstallation of Exchange 2016 you are presented with the following error:
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The reason is that in Exchange 2016 you now have an Audit Mailbox, you can check this by running the following command:
- Get-Mailbox -Auditlog
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As you can see in the last command we have a system mailbox, you can now move it to another server as shown below:
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You can run the new-move request command to move it to another store and then you can proceed with your cleanup or removal of Exchange 2016.
Hope it helps.