Single opt-in is a common procedure used in email marketing where the user will receive a single confirmation message when he subscribes to a newsletter. A single entry to the database is made after the single opt-in procedure. Affiliate marketers use email marketing effectively to keep track of the website visitors. In the subscription form, the single opt-in process will send a confirmation email confirming the registration on the website. Another procedure that is commonly used now is double opt-in where a second email requiring explicit confirmation is sent. This is to ensure that the user can protect his data if he wants to.