The term opt-in refers to the act by which a user agrees to be included in a distribution list to be contacted via email or other forms of communication. If once you register in a contact list you receive a communication in which you have to click to make this registration effective, we are talking about a double opt-in.
Unlike promotional spam emails, which are sent to a long list of recipients regardless of whether or not they want the information, opt-in e-mails are sent only to people who specifically request it.
The opt-in has as antonym the opt-out, which is the action carried out by a user to leave a list of contacts.
Opt-in in Email Marketing
In Digital Marketing, opt-in e-mails are targeted and often personalized, and deal with information about specific topics or promotions that users are interested in knowing. Typical opt-in emails contain newsletters, product information, or special promotional offers. For example, if a user frequented a Web site that sells books and music online, that user may “opt- in” to receive ads when their favorite musician or author releases new material. The promotional email may even present the recipient with a special promotional offer to purchase the product at a discount available only to those present on the opt-in list.
Opt-in and spam
Spammers, however, have found a way to use the opt-in feature to the benefit of spam. Spam emails often come with opt-in features in small print or obscured somewhere in the body of the email, with text saying something similar to “failure to respond to this email address will be understood as the recipient’s approval automatically to accept future submissions.” The strategy of responding to spam and asking to be removed from future shipments also has critics, as they claim that responding to a spam email can cause that address to be transmitted to other spammers.