We spend six figures a year on several data partnerships. We also crawl the web and social sites ourselves. We then pull everything together, match records, then use our email validator to double verify emails.
Most domains are "catch-all", and we can't verify those. We consider these "Single Verified". It means that we came across it while we crawled the web or from a data provider. You'll see these single verified emails with a bounce rate around 20%.
"Double Verified" means we were able to confirm by connecting to the server and validate the inbox exists.
In addition, we now validate these again right when they get added to your list. Provided you contact them shortly after you add them, then you'll see less than 1% bounce rate on those.