A term that refers to promotional e-mails that have been requested by the individual receiving them. Unlike spam promotional e-mails that get sent out to large lists of recipients without regard to whether or not they want the information, opt-in e-mails are only sent to people who specifically request them.
Opt-in e-mails are targeted and often personalized and carry information about specific topics or promotions that users are interested in learning about. Typical opt-in e-mails contain newsletters, product information or special promotional offers. For example, if a user frequented a Web site that sold books and music online, that user could "opt in" to receive announcements when his favorite author or musician released new material. The promotional e-mail may even present the recipient with a special promotional offer to purchase the product at a discount available only to those on the opt-in list.
Spammers, however, have found a way to use the opt-in feature to their advantage. Spam e-mails often come with opt-in features in fine print or obscured somewhere in the body of the e-mail with text that reads something to the effect of "Failure to respond to this e-mail will automatically opt-in the recipient to future mailings." The strategy of responding to the spam and asking to be removed from future mailings also has critics as some claim that responding to a spam e-mail with a request to be removed from the mailing list signals that the e-mail address is a working address and the address can then be passed on to other spammers.