Opt-in

Definition:

In e-commerce, when a user signs up to receive e-mails about products and services that are of interest to them; also used to refer to the mailing list itself.

Examples:

Customers are more likely to opt-in to a mailing list if they are promised discount coupons as well.

Once people opt-in, they seldom unsubscribe, and will continue to receive the marketing messages without any further action by the company.