A double opt-in is one of the sign-up methods for receiving email marketing. The first step is when a user submits their email address while subscribing to, for instance, a newsletter. The next step is to activate the subscription by confirming the email address in the received message.
One of the benefits of the double-in method is that it lets you create a valid mailing list of high quality audience. On the other hand, because users are asked to perform another Action, it’s possible that they would overlook your confirmation email and you won’t be able to add them to your list.