I was hoping that this did something a bit more worthy of Hacker News, like checking if the A Record exists. Or perhaps speaking SMTP to the domain in question to query if the user actually exists (works on some domains) using web sockets.
Strictly, you might need to check for both. If there are no MX records, mail will be delivered to the @ A record for a domain (e.g. mail to x@example.com will go to the IP for 'example.com').
So if there are no MX records, you need to check @'s A record as well (or vice versa, since not all domains have A records either).
http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-...
Maybe I'll build it for fun.