Buttondown Documentation
Double opt-in is a process that requires a subscriber to confirm their subscription before they can receive emails from you. It's a way to ensure that subscribers are actually interested in your content.
Single opt-in, conversely, does not require subscribers to confirm their subscription before they can receive emails from you. Single opt-in is allowed in certain use cases on Buttondown (for instance, if your subscribers are confirming their subscription on another platform such as Shopify or Stripe), but cold emails is not.