There are several common forms of opt-in e-mail:Unconfirmed opt-in
Some marketers call closed loop opt-in "double opt-in."
The term double opt-in was coined by marketers in the late 90s to differentiate it from what they call single opt-in, where a new subscriber to an e-mail list gets a confirmation e-mail telling them they will begin to receive e-mails if they take no action. This is compared to double opt-in where the new subscriber must respond to the confirmation e-mail to be added to the list.
Some marketers 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.
The term double opt-in has also been co-opted by spammers, diluting its value.Opt-out
E-mail authentication is a technique for validating that a person claiming to possess a particular email address actually does so. This is normally done by sending an email containing a token to the address, and requiring that the party being authenticated supply that token before the authentication proceeds. The email containing the token is usually worded so as to explain the situation to the recipient and discourage them from supplying the nonce (often via visiting a URL) unless they in fact were attempting to authenticate.
For example, suppose that one party, Alice, operates a website on which visitors can make accounts to participate or gain access to content. Another party, Bob, comes to that website and makes an account. Bob supplies an email address at which he can be contacted, but Alice does not yet know that Bob is being truthful (consciously or not) about the address. Alice sends a token to Bob's email address for an authentication request, asking Bob to click on a particular URL if and only if the recipient of the mail was making an account on Alice's website. 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 register for an account because the confirmation would be sent to Bob's email address, which Chuck does not control.