Hello,
Alternatively you can also associate the mailbow with another user inside Active directory, however the new user should not have an associated maibow within Exchange.
Refer to the following technote which suggests some more workarounds:
Title: After deleting a user in Active Directory, the error message "Directory not found. Cannot backup directory and its subdirectories" appears when backing up the user's mailbox.
http://support.veritas.com/docs/241981
Hope the above info answers your question. Update in either case.
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Regards.