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.

Building and managing an opt in list for a website
Building and managing an opt in list for a website provides a seller direct access to a wide number of potential customers. Building and managing