Express instruction by a customer, or a recipient of a mail, email, or other direct delivery to stop the marketer from sending a merchandise, information, or more messages. It is a type of program that assumes inclusion unless stated otherwise.
See also "Opt-in".
For example, some book clubs automatically send the 'Book Of The Month' to their members unless they notify the club (within a specified period) not to send it. Responsible online marketers include opt out instructions in their messages or at their sites.
Opt-out is sometimes used to refer to email which assumes inclusion; sometimes having “Yes” automatically checked on a signup form, sometimes by adding your name directly to a list.1)