Why do you think that matters? The sending site knows if it sent an email. The only thing it doesn't know is if you received it. For an account registration or

Why do you think that matters? The sending site knows if it sent an email. The only thing it doesn't know is if you received it. For an account registration or password reset email, the sending site will typically send you an email with a link inside it containing a secret token. If the JavaScript reads that it received an email with one of those links in it, it will post that information back to the server so the server can complete the function it started (account registration or password reset). Obviously, at that point, it will know if the token it was passed was the one it sent. [edit] Also, as this system is being implemented from scratch, it could easily be decided that emails which don't pass DMARC for example will simply be ignored. Although I argue that is not necessary.