That does not match my experience either. In fact I don't see why any mail server would bother getting and configuring an IPv6, listen on it, just to drop all r

That does not match my experience either. In fact I don't see why any mail server would bother getting and configuring an IPv6, listen on it, just to drop all received messages. Just don't listen on v6, that would achieve your goal, would prevent reputable dual-stack senders (like Google!) to send messages over v6 that you'll drop, and simplify your configuration.