Unable to Delete Exchange 2003 Mailbox Store

Several times during Exchange 2003 to Exchange 2010 migrations, we've run into an issue where we are unable to delete the last mailbox store before uninstalling Exchange 2003. We receive a message like:

One or more users currently use this mailbox store. These users must be moved to a different mailbox store or be mail disabled before deleting this store.

ID no: c1034a7f
Exchange System Manager

However, when we use System Manager to view the list of mailboxes, only system mailboxes are displayed. The system mailboxes do not need to be deleted before the mailbox store can be removed. There are references to the mailbox database in Active Directory that may or may not be valid.

The easiest way to identify the user accounts with references to this database is by using a query in Active Directory Users and Computers. Create a custom query and enter in the following LDAP syntax:

(&(mailnickname=*)(msExchHomeServerName=*OldServerName*))

This query search identifies mailboxes hosted on OldServerName. Since we are down to a single mailbox database on that server, it is unique enough to get us the information we need. Microsoft also has a list of other methods at http://support.microsoft.com/kb/279202/en-us.

After you identify the users with invalid references, you need to remove the references. In some cases, you can delete the mailbox from the user in Active Directory Users and Computers. Other times, you need to remove Exchange attributes from the user.

After you have cleaned up the Exchange information in the user accounts identified by your query, you should be able to delete the mailbox store and complete removing Exchange 2003.