That's what happens now, but it's really not great UX. Typically by the time the user reads the modal dialogue telling them not to close the window, the email

That's what happens now, but it's really not great UX. Typically by the time the user reads the modal dialogue telling them not to close the window, the email will be sent. The page then needs to make clear "hey that thing we just said? You can ignore it now". I appreciate the suggestion, but it's just a very confusing and disruptive approach to solving the problem.