How do I check if an email address is valid without sending anything to it?

Question

I have a client with 5000 emails from an old list he has that he wants to promote his services to. He wants to know which emails on the list are still valid. I want to check

Answer 1

https://github.com/kamilc/email_verifier is a rubygem that will check that the MX record exists and that the SMTP server says the address has a valid mailbox.