When someone freely signs up to a mailing list, it is called as Opt-in. Double Opt-in is just a little bit different. It is a method of requiring email subscribers. You send them an email with a confirmation link and they sign up by clicking on it. If he does not confirm the subscription, he will not receive an email newsletter.
This practice can not be used forever for marketing. The double opt-in usually causes a subscriber loss. To decrease and avoid the loss, it is necessary to make a need to confirmation clear in text and email subject.
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