Double opt-in is a process through which a new subscriber must confirm their subscription before being subscribed to a given list, via email or SMS.
When you add a signup form to your site, you are not able to control who decides to take advantage of this form. Even if you add an extra level of validation, it isn’t always possible to ensure subscribers only submit valid or accurate email addresses and phone numbers. Well-intentioned subscribers could simply type their email or phone number incorrectly, but spam-bots could also find your form and flood it with fake email addresses or phone numbers.
The double opt-in process helps you grow your list while also minimizing abuse and preventing the accumulation of invalid or mistyped emails and phone numbers. For email in particular, this is important because most major email clients (like Google and Yahoo) track how recipients interact with emails from your domain — how many are marked as spam, how many are opened, how many bounce, etc.
They use this information to determine whether your emails are classified as spam. Having a lot of uninterested people or a lot of invalid emails on your list will only hurt your efforts to reach those that actually want to receive your emails.