Opt-in email is a term used when someone is not initially added to an emailing list and is instead given the option to join the emailing list. Typically, this is some sort of mailing list, newsletter, or advertising. Opt-out emails do not ask for permission to send emails, these emails are typically criticized as

unsolicited bulk email Email spam, also referred to as junk email, spam mail, or simply spam, is unsolicited messages sent in bulk by email (spamming). The name comes from a Monty Python sketch in which the name of the canned pork product Spam is ubiquitous, unavoida ...

s, better known as spam.

Forms

There are several common forms of opt-in email:

Unconfirmed opt-in/single opt-in

Someone first gives an email address to the list

software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...

(for instance, on a Web page), but no steps are taken to make sure that this address belongs to the person submitting it. This can cause email from the mailing list to be considered spam because simple typos of the email address can cause the email to be sent to someone else. Malicious subscriptions are also possible, as are subscriptions that are due to spammers forging email addresses that are sent to the email address used to subscribe to the mailing list.

Confirmed opt-in (COI)/double opt-in (DOI)

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''. Some marketers call closed-loop opt-in "double opt-in". This term was coined by marketers in the late 90s to differentiate it from what they call "single opt-in", where a new subscriber to an email list gets a confirmation email telling them they will begin to receive emails if they take no action. 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. However, it does drastically reduce the likelihood of someone being signed up to an email list by another person. 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. The