Kajabi Double Opt-In is a feature related to email marketing that enhances email list security and engagement. Double opt-in is a two-step process in which subscribers confirm their email addresses by clicking on a verification link sent to their inbox. Kajabi users can enable this feature, ensuring that only verified subscribers are added to their email list. It reduces the likelihood of fake or mistyped email addresses being added and strengthens the quality of the list. Additionally, double opt-in can lead to better engagement, as subscribers have already expressed active interest in receiving content from the sender. This feature aligns with best email marketing practices and helps maintain a healthy and engaged email audience.