I don't know how
this system does it, but I've done this with FullContact before (and
https://clearbit.com/ will do it too). I have a few hundred thousand people and get about 50-60%.
Recently we tried using this data in the "To" field of emails we sent but it actually caused open rates to go down 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.
