Double opt-in is a process where subscribers are required to confirm their subscription by clicking on a link sent to their email. This extra step ensures that the email addresses collected are valid and that the subscriber genuinely wants to receive emails from you. Implementing double opt-in can reduce the number of spam complaints and improve your email deliverability rates. It also helps in maintaining a high-quality list with engaged subscribers, which is more valuable than a large list of uninterested recipients.