For a brief moment, there was a service (Earn.com/21.co) that did this. It was a company that had previously made a SoC mining rig for crypto and the pivoted to this thing where people would pay you to open their messages. Eventually it was overrun by spam messages from scammy crypto ICOs and there were conditions on payouts where you had to sign up for an external email list or register for an airdrop. I think they were bought and shut down by Coinbase.