Credit for design goes to Vivek Venkatraman: http://dribbble.com/shots/1455744-Email-Concept/
Check it out in full view to see it's responsiveness. It's responsive until pretty darn small but i haven't actually tested it on mobile devices. Tested in IE9+, latest Chrome and Firefox.
I've noticed the message (slide-in section) is some times visible on page load. If that's the case just reload the page, that's a browser bug.