Opt-in email is a Web marketing term for email that recipients have previously requested by signing up at a Web site or special ad banner. Typically, Web users are invited to sign up for promotional information about one or more categories of products or services. Those who sign up have thus "opted in." Anyone sending them email as a result hopes that the message will not be perceived as unwanted spam.
Several companies gather sign-ups at their own site or through specially-designed banner ads and then sell marketers mailing lists of those who have signed up in various interest categories. The marketer sending opt-in email may remind the recipient that they have previously indicated they were interested in receiving such email and that this is not spam. The recipient is given an opportunity to be removed from the mailing list if they so choose. The distribution model of sending unsolicited email (spam) and allowing the recipient to request removal is sometimes referred to as "opt-out."
Opt-in email has been endorsed as the best practice for marketers by the Internet Direct Marketing Bureau (IDMB).