I am not an admin, but, to the best of my knowledge, you cannot "retrieve" a deleted user.
You can, however, recreate a new ID, which will entail creating a new mail file. Delete the new mail file, and point the new Person document to the "old" mail file.
Be sure to assign the user as Owner of the mail file in the ACL and in the user profile document.
When that is done, all should work well.
Pascal.