Ah, this concept reminds me of one of the first apps I did, years ago, called EmptyInbox - https://itunes.apple.com/us/app/emptyinbox-for-gmail/id50994... (Yay skeuomorphism!)

It only supports GMail, but still works as far as I know. Back then it was the only app I knew that had that concept, but by now there is Triage, plus most modern mail clients have the "swipe" actions for quickly getting through emails.