The good news is seeing who has unsubscribed from your contacts list is available in your account! You can see the total list of unsubscribed contacts through the contacts tab of our account.
You can also see them on a case by case basis through the Unsubscribed Rate column on the Reporting tab of your account.
Constant contact even offers the ability to receive email updates when they have unsubscribed. You can customize how often you receive these emails too.