Why? You will want to, from time to time, to validate your address book and scrub it against the Stop Log.
What is the Stop Log?
The stop log is a report of all users on your account that have decided to no longer receive text messages and have replied 'STOP.'
How Do I Do This?
From your 2sms portal, select the button Validate Address Book inside of the Address Book field. Then you will select Scrub Address Book Against Opt Out Table. This will provide you with a list of users that are on the Stop Log. You will then select the Delete Mobile Numbers from Address Book to remove those contacts from your address book.