Yup. Just requesting the sender to solve some riddle (and waste their energy in process) would turn the tables completely because the cost of sending would be n

Yup. Just requesting the sender to solve some riddle (and waste their energy in process) would turn the tables completely because the cost of sending would be non-zero. Unfortunately it would also mean that we would be sacrificing our planet again. But maybe the difficulty of the challenge could adapt according to some trust score?