This has come up so often on Hacker News that I decided to create a very simple JSON API for checking email addresses. Free to use for anyone. Performs the right regexp check for email addresses based on RFC-5321 rules (not the oft-quoted but incorrect RFC-822 rules, which are for mail headers), performs MX lookups to ensure mail can be delivered, and performs the same "did you mean" type checks that kicksend's mailcheck performs.
I've included both jQuery and server side example code on the site.
The domain outlok.com has an A record which points to 208.87.35.108 which I can even connect to on port 25 (though I don't perform that test). Until you actually mail that address there's no way to distinguish this from a valid email.
I've included both jQuery and server side example code on the site.
https://www.emailitin.com/email_validator