Ran into this even though there wasn’t any mailboxes (as far as I could see)
Used this method
Method 4: Use Active Directory Users and Computers to search for mailboxes that are on a mailbox store
- Start Active Directory Users and Computers.
- Right-click the domain that you want, and then click Find.
- Click the Advanced tab, click Field, point to User, and then click Exchange Home Server.
- In the Condition list, click Ends with, type the name of the Exchange computer, and then click Find Now.
- If you are prompted to add the current criteria to your search, click Yes.
- When you have identified which users have mailboxes on this store, you can use Active Directory Users and Computers either to move the mailboxes to a different store or to delete the user’s mailbox.
The mailbox search results appear in the bottom pane.
From here to see that there was still one mailbox left.