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.
contend that "double opt-in" is like asking for permission twice and that it constitutes unnecessary interference with someone who has already said they want to hear from the marketer.
Double opt-in method is used by email marketers to ensure the quality of their list by adding an extra stop in the verification process.
For example, suppose that one party, Alice, operates a website on which visitors can make accounts to participate or gain access to content.
Bob receives the mail and clicks the URL, demonstrating to Alice that he controls the email address he claimed to have.
If instead a hostile party, Chuck, were to visit Alice's website attempting to masquerade as Bob, he would be unable to complete the account registration process because the confirmation would be sent to Bob's email address, to which Chuck does not have access.