The user types their address, hits submit, and walks away. They meant zhang@gmail.com . What they submitted was zhang@gmail.co . The form accepts it, because gmail.co is a syntactically valid domain. The confirmation email sails off to an address no one owns. The user never arrives, never complains, and you never learn why. This is the quiet leak in most signup flows. It is not a bounce you can measure later. It is a silence you never see. What a typo correction actually does ...