The best idea ive seen is if you decide what a user sees in an algorithm way (beyond “natural” ones like chronological) you are liable for the content - you are no longer a common carrier and can be sued for damages. That should be step 1.
So a mail s/w that pushes up mail that it deems important?
Modern social media is choosing from a vast pool of content and the algorithm's entire goal is to maximize engagement for monetary gain (at whatever cost to society). There's only so much "engagement maxing" you can do with an email sorting algorithm especially if the premise for an email inbox is that you'll always only ever see your emails and nothing else.