Double opt-in is a measure that ensures that the user really wants to be subscribed to your mailing list, newsletter, or some other method of email marketing. In order to confirm their decision, the users are required not only to subscribe to your service but also to click the link in the confirmation email. Only then the advertisers have the right to start sending promotional emails to the subscriber. Double opt-in is an option for both advertisers and users. From the advertisers point of view, this method is a way of ensuring that their message will reach the right people, thus eliminating the risk of getting flagged as spam. When it comes to users, double opt-in prevents them from subscribing by accident.