I'm not sure since when, but http://mailtracking.com/ has been around for ages and they have funnier versions of email too.. including a "mission impossible" count down version.. (i'm amused but i don't work there)
anyways, as mentioned above these methods aren't 100% reliable (less than 100% just defeats the purpose right?)... so, didtheyreadit.com is not correct.. but the more important question is.. did they NOT read it?