It looks like where you went wrong was replying with a screenshot of your calendar. That and the likely passive aggressive text accompanying it was probably a red flag for Jacob. The fact that you posted this drivel vindicates him.
I never harassed him about a return receipt. I just get to see when he reads my emails so I know how long it takes him to respond.
It's a feature of Superhuman. Controversial for sure, but you can even enable them in Gmail. Honestly, a feature built into every Enterprise email software out there. Not uncommon at all.