I don't know how <i>this</i> system does it, but I've done this with FullContact before (and <a href="https://clearbit.com/" rel="nofollow">https://clearbit.com/</a> will do it too). I have a few hundred thousand people and get about 50-60%.<p>Recently we tried using this data in the "To" field of emails we sent but it actually caused open rates to go <i>down</i> for some reason, so still figuring that one out. Investigations with a few users showed using the full envelope caused emails to go into different inboxes than were expected, but if you can do it from the get go it would be a great technique.