Opt-out, opt-in and double opt-in approaches in creating a mailing list are possible.

Opt-Out

An opt-out list is one to which email addresses are added without the user's knowledge or permission. At best there was a prior relationship between the sender and the recipient. At worst it is a pure spam.

Obviously an opt-out list has the lowest response rate and the highest complaint rate.

Confirmed Opt-Out

A confirmed opt-out list is one that sends a confirmation to intended recipients when their email addresses are added to the list. This confirmation usually lets the recipient know that his email address was added to the list and allows him to unsubscribe by replying to the email or clicking a link within the body of the message.

A confirmed opt-out list can be very dirty too if the user thinks the confirmation letter is a spam and does not bother to reply to this message.

Opt-In

Perhaps this is the most straightforward one of all lists. An opt-in list is one with which users must take some action to subscribe. Typically, this involves filling out a Web-based signup form. Surely, a list consisting of people who actively add themselves to it (as opposed to a list of people who passively allow themselves to be added) is more responsive and produces fewer complaints.

The problem with an opt-in list is that third part persons can (and they do) add others to lists.

Confirmed Opt-In

Confirmed opt-in lists confirm, by email, your subscription as soon as your name has been added to the list. They allow you to unsubscribe immediately by replying or clicking on a link within the email, for example like this one:

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You receive this email because you or someone else added your email address to our mailing list. If you would prefer not to receive this newsletter in the future, please, reply with the word Remove in the Subject line, or click on the link below:

remove@domain.com?Subject=remove

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In this case, even if someone signs you up for a bunch of newsletters you don't want, you're alerted immediately and can take the appropriate measures.

Double Opt-In

A double opt-in list means not only must the user take an action to add himself to a list, but he then receives a confirmation of his subscription. He must reply to be added to the list.

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This message is to verify that you wish to have your email address [%Email%] added to [%Name%] newsletter

You MUST reply to this message to have your email address added to our list.

This is to ensure that someone doesn't add your address to our list without your prior knowledge or consent.

Unless you respond to this email, your address is not added to our mailing list!

Thanks,

Customer Service

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This is done to ensure that the customer did not subscribe by mistake or somebody else did not subscribe him to receive your regular email. Even if a third part person attempts to add somebody without his permission, that confirmation, that second step requiring action, means if the customer does nothing he won't be added to the list. Requiring two active and intentioned steps makes for the cleanest list possible. Raising the bar and requiring two actions means the highest response rates and the lowest complaint rates.

Email Processor provides you an example of My Subscribers rule to handle emails with subscribe requests and automatically send a confirmation letter to your customers. To process emails from your customers confirming that they wish to be added to your mailing list, you can use My Subscribers Confirm rule.