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

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1. Start Active Directory Users and Computers.
2. Right-click the domain that you want, and then click

   Find.

3. Click the Advanced tab, click

   Field, point to User, and then click

   Exchange Home Server.

4. In the Condition list, click Ends

   with, type the name of the Exchange computer, and then click

   Find Now.

5. If you are prompted to add the current criteria to your

   search, click Yes.

6. 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.

http://support.microsoft.com/?id=279202

Notes