Mailboxes in Exchange 2000 are attributes of user accounts. You should enable auditing in your Active Directory and track all changes to the accounts. In the future, this will cause an event to be logged thus telling you what accounts have been changed (in this case, whose mailbox was just deleted). As for what to do now, I am not aware of any third-party tools that specifically have this feature, but that does not mean there are none. You might try perusing through the offerings here. Perhaps one of these vendors has a solution for you.
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