The Problem:
You have an employee that has left your company. Before leaving he/she has deleted their email. As your company has a backup system - you are not worried.

Well you should be !
Most backup systems take a snapshot of the Email system as it is currently. The problem is when a user deletes his/ her email and the next backup is run - the result is in that the email is now deleted from the backups too !
This means that you are not able to retrieve that person's email ever again !