An email list is simply a list of at least email addresses. A permission-based list however, indicates that list members have actively requested to have their email address added to a list.  List members often subscribe by entering their contact information into a  contact form on someone's website. 

The term "double opt-in" means that the subscriber received a confirmation email when the  contact form was filled out and that they confirmed their new subscription by clicking on a link. Failure to implement permission-based lists or to keep clean lists (quickly removing invalid email addresses) may result in the sender being labeled as SPAM.

Permission-based email marketing is a best practice in the industry, delivering higher open, click-through, and conversion rates.

BombBomb.com provides a rich set of tools for building and maintaining permission-based lists including contact form creation, list management, list cleansing and subscriber management features.