A list of email addresses you have removed from your regular mailing lists, either because they have opted out of your lists or because they have notified other mailers that they do not want to receive mailings from your company. Required by CAN-SPAM. AKA Do-Not-Email list.
Are you successful with your Internet business? Better yet, do you think you are successful with your online business? When most online entrepreneurs are asked with these questions, they often brush it off.