On a sidenote: Even if you the MX entry is valid, the mailserver is reachable and you are even to validate the email address by sending one one those "click here to validate" links, you can't besure it will work for more than a few minutes. There are more and more of this one-time email-address services available to avoid SPAM and all these annoying "you have registered at our site a few years ago so we constantly bombard you with our 'newsletter'" thingies.

I'm currently teaching my private mailservers how to do that onetime address thingy for the same reasons...