The CAN-SPAM Act requires that users be allowed to opt-out of an email marketing list.  Keeping 'clean' permission-based lists by making opt-out's easy is also a very good practice to follow.  

Users can subscribe to lists using BombBomb's contact web-forms. Custom subscription confirmation emails can be automatically sent so that recipients can double opt-in. When a recipient of an email from BombBomb decides they no longer want to receive the emails they subscribed to, they can opt-out by selecting an unsubscribe link at the bottom of the email.

Unlike other systems, BombBomb will display all the email lists from the sending organization that the recipient has subscribed to. By doing so, the recipient can easily manage their subscriptions. This makes opting-out easy for the recipient especially when the sending organization has included them in multiple, segmented lists.

For example, if the recipient no longer wishes to receive emails from NotSoTastyWineCorp.com, it would be helpful to have their name removed from all of the companies lists at once including the Red, White, and Sparkling email lists.