For the sake of quality (but also to follow the law), your mailing lists must qualify as opt-in. Before you learn more about double opt-in, make sure you understand what opt-in means. It's important that you understand this concept. Quality mailing lists are fundamental to your deliverability.
Definition of double opt-in
Double opt-in means sending a confirmation email to the person who subscribes to your mailing list. It is an even more qualitative way to collect email addresses. You make sure that the email exists, and that its owner wants to read your communications.
Let's say, for example, that we qualify our newsletter subscribers as double opt-in. This prevents an increase in the bounce rate. We assume that if you want the content of that weekly newsletter.. then you'll take the trouble to confirm your address!
Is simple opt-in not enough?
We often hear that verification is dangerous. Why? Because it requires an extra step from the user. That it will make you lose subscribers. Sending an email to confirm the subscription for other emails may seem unnatural.
And yet, think of all the advantages of this solution
You get the certainty that the email address exists. This reduces the potential for bounces — which is great for your deliverability. It helps the reputation of your IP/domain. You'll have a very high open and click rate in the email (during confirmation). This will send a very good signal to email services and spam filters.
You are sure that your subscribers have really asked to join your list. By setting up a double opt-in system, they can't say they didn't subscribe or that their address was taken.
Moreover, things tend to improve as double opt-in has become common. Thanks to Google's Email Markup project, confirmation can happen from the inbox. It doesn't even need the email to be opened!
Are there any disadvantages?
There are few disadvantages with double opt-in. Some will say that we may miss subscribers. We reassure you, this part is negligible. All you need to do is be clear during the subscription that a confirmation email has been sent.
People are used to this kind of process. Don't worry about it. Instead focus on a solid and qualitative email acquisition strategy. In any case, a person who doesn't confirm their registration doesn't have a strong desire to receive communications from you. They're unlikely to open later emails.
Double opt-in for all situations?
We say to always think about double opt-in and to apply it to all your address acquisition strategies. Whatever the nature of your mailing list. It works for newsletter subscribers, accounts created on your service, etc.
Depending on the project, it's not always easy to qualify your contacts with double opt-in. The methods used to collect email addresses can also impact your ability to start with double opt-in.
If you don't wish to double opt-in, you still must follow the opt-in rules. And have a dated written proof of registration to your list.