A single mailbox was deleted from your Exchange server and is not available using deleted mailbox recovery; you have a backup from the time frame before the mailbox was deleted.
Prior to beginning the recovery process, check to see if you have set a retention period for deleted mailboxes:
Launch Exchange System Manager.
Browse to the mailbox store where the deleted mailbox used to live.
Right-click the mailbox store and select the Properties command.
Click the Limits tab. If the value in the Keep Deleted Mailboxes for (days) area in the Deletion Settings portion of the tab is greater than zero, and the mailbox was deleted within the number of days shown, you can use the method in Recipe 11.10 to retrieve the mailbox. The default deleted mailbox retention time in Exchange 2000 and 2003 is 30 days.
If the mailbox is not recoverable through deleted mailbox recovery steps, there are a few more hoops to jump through. If you are running Exchange Server 2003, follow the instructions in Recipe 11.11 to restore the backup to the RSG. If you are running Exchange 2000, follow the instructions in Recipe 11.6 within a non-production environment, including a domain controller moved from production into the offline environment. Then you can proceed to actually recover the mailbox.
Create a user account in your nonproduction AD with the Active Directory Users and Computers snap-in; when prompted, ...