Opt-in email is one of the most important internet marketing terms. It is used for the cases in which the users are given the choice whether to accept receiving emails from an advertiser or not. If they accept, they will start receiving promotional emails that contain special offers, news about the company, and so on. If they dont accept it, the emails sent by that advertiser will be regarded as spam. That is why its extremely important for the advertisers to provide the visitors with a subscription option. Basically, if a user chooses to accept receiving newsletters, they will have to confirm their decision by clicking the link found in the opt-in email.