A Robinson list or Mail Preference Service (MPS) list is an opt-out list of people who do not wish to receive marketing transmissions. The marketing can be via e-mail, postal mail, telephone, or fax. In each case, contact details will be placed on a blacklist. Although the purpose of such a list is to reduce unwanted marketing, the principle that "data is only private until someone decides it isn't" indicates it can also generate unwanted marketing.