Free Email Address Verifier:

If you need to verify someone’s email address, try Verify Email Address Tool - a free email address verifier. Just enter in the email address and click “Verify”. The format, domain, and user are all checked by actually connecting to the mail server.
This email verification tool actually connects to the mail server and checks whether the mailbox exists or not.

What is being verified:
Format: "name@domain.xxx"
Valid domain: "somebody@new.york" is not valid
Valid user: Verify that the user and mailbox really exist
After Verification u will see this thing:
Result: Ok
Log:
MX record about gmail.com exists.
Connection succeeded to alt3.gmail-smtp-in.l.google.com SMTP.
220 mx.google.com ESMTP g4si19308825ick.85
> HELO verify-email.org
250 mx.google.com at your service
> MAIL FROM: <check@verify-email.org>
=250 2.1.0 OK g4si19308825ick.85
> RCPT TO: <xxxxxxxxxxxxxx@xxxx.com>
=250 2.1.5 OK g4si19308825ick.85