Customers may sign up for mailing lists out of their free will. All they have to do is click on a subscription offer and there are immediately added to the list of recipients. This process is called opt-in.
There is a special form of opt-in called Verified opt-in or Double opt-in. It consists of two steps. First, you have to sign up for subscription as in normal opt-in. The subscription, however, needs to be verified by clicking a link in an email. This process is meant to prevent spam. Some countries require this verified form of opt-in by law.
From Starting Your Own Affiliate ProgramGet Started