In the event of a disaster where an Exchange mail store is destroyed or damaged it is possible to have Active Directory accounts that are configured as Exchange mail users but that point to invalid (non existent) mail stores.  The DiRT restore utility is not able to determine the mail store they are pointing at is not really there, it will assume the user is a valid mail user and update their account to be a Unity subscriber.  However, the user will not be a functional subscriber and, in short, the restore for this user will fail.  Even if you update their AD account to be properly mail enabled the Unity data will not “stick” through this process and you will lose subscriber data for that account.

To avoid this you can do one of two things:

1. Mail enable the AD accounts prior to running the DiRT restore utility.  DiRT will bind to this account properly during restore and everything will be fine. 
2. Delete the AD accounts that point to invalid mail stores entirely.  DiRT will recreate both the AD and mail store data for these users and update them to be Unity subscribers automatically during the restore process.