Maintaining good list management is a key element in keeping your users happy. Sending email to someone who has unsubscribed or never signed up in the 1st place is a big no-no in the email game.
Make sure to check your list stats. You can do this in Marketing Campaigns by clicking on the stats button next to your recipient list name. You can also manage your unsubscribes by clicking the manage unsubscribes button next to your recipient list name.