Is there a reason to do something like this using a bounce address? I mean, that's what at least all the big players seem to be doing, but couldn't you just as well use something like A PING [1] or some sort of Ajax pingback? There's browser compatibility of course, but supporting old IEs shouldn't be critical for something like this.

[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#...