Several improvements

I've made several additions that might be useful:

*    Doing a simple regex check before starting a connection
*    Handle MX lookup failures
*    Send full HELO message when making a connection, since some servers don't respond otherwise.
*    Handle various connection issues(timeout, connection refused etc.)
*    Added an `undetermined` argument to the callback, since you might want better distinction between a definitely non-existent email and one that can not be verified.

P.S.: I apologize for the sloppy pull-request, I wasn't planning on sending you one when I created the fork.