Double opt-in is a process in which a user must confirm their subscription or consent twice before being added to a mailing list or receiving marketing communications. This method ensures that the user has given explicit and deliberate consent to receive emails or messages, providing an extra layer of protection against spam and unwanted communication. It typically involves the user first signing up through a form or website, and then receiving a confirmation email with a link or code to verify their subscription. This two-step process not only ensures compliance with privacy laws and regulations, but also helps to maintain a high-quality and engaged audience for businesses and organizations. Double opt-in is considered a best practice in email marketing and is widely used to build trust and credibility with subscribers.