A new subscriber asks to be subscribed to the mailing list, but unlike unconfirmed or single opt-in, a confirmation email is sent to verify it was really them. Generally, unless the explicit step is taken to verify the end-subscriber's e-mail address, such as clicking a special web link or sending back a reply email, it is difficult to establish that the e-mail address in question indeed belongs to the person who submitted the request to receive the e-mail. Using a confirmed opt-in (COI) (also known as a Double opt-in) procedure helps to ensure that a third party is not able to subscribe someone else accidentally, or out of malice, since if no action is taken on the part of the e-mail recipient, they will simply no longer receive any messages from the list operator. Mail system administrators and non-spam mailing list operators refer to this as "confirmed subscription" or "closed-loop opt-in".