How do we validate an email address?

This article explains how we validate emails usin the email server response to the email address.

Essentially we have a conversation with the email server behind the email address to try and understand how that server is operating and how it responds to the email address. We do this without actually sending an email.
 There are challenges to do this at scale, to handle non-conformance with standards, and to handle other obstacles. The measures we use to resolve these challenges are our intellectual property and therefore cannot be shared.
 If you would like more information on how the basic process works our partner's free tools site has this article: How to Ping Email Addresses.