Email Pie is a wonderful little JSON API that gives you a simple way to validate email addresses. We'll let you know if that email address is legit or not.
Well, first we make sure the email is formatted correctly. Catching things like invalid characters or no @ signs. Then, we make sure that the domain is configured to collect email. Finally, we check for common misspellings.
It's free for now, but we limit API calls to 1800/hour. Or, run your own.