Email Opt-in is a term used when someone is given the option to receive "bulk" email, that is, email that is sent to many people at the same time. Typically, this is some sort of mailing list, newsletter, or advertising. Without obtaining permission before sending email, the email is unsolicited bulk email, better known as spam.
Unconfirmed Opt-in:
Someone first gives an email address to the list software (for instance, on a Web page), but no steps are taken to make sure that this address actually 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:
A new subscriber asks to be subscribed to the mailing list, but unlike unconfirmed opt-in, a confirmation email is sent to verify it was really them. Many believe the address must not be added to the mailing list unless an explicit step is taken, such as clicking a special web link or sending back a reply email. This ensures that no person can subscribe someone else out of malice or error. 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. The term double opt-in has also been co-opted by spammers, diluting its value.
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