Pretty sure I’ve seen a system like this in action. People sign up for our service; we send them a receipt (and for many of them, the cost is tax deductible); a

Pretty sure I’ve seen a system like this in action. People sign up for our service; we send them a receipt (and for many of them, the cost is tax deductible); and it bounces with a message that we have to click on a link to enable it to go through. (For a number of reasons, we don’t. And most companies send stuff like that from a black hole no-reply address anyway.) Good luck when your bank tries to contact you about something important but the email bounces.