Um, sorry we don't have this (there is a "Mark all read" in smart folder, but apparently it doesn't do what you want).
I'd try app settings -> notifications -> notify about -> set to "new" messages.
This way if you swipe off a notification -- the app will remember it as a "checkpoint" and won't notify about same messages again.
And then when your "truly important" messages arrive -- it'll be easier to see that.