It kinda does, it's called greylisting and already works today: the recipient basically says "can't right now, please come back later". A spammer will not bothe

It kinda does, it's called greylisting and already works today: the recipient basically says "can't right now, please come back later". A spammer will not bother even reading this and will hit other targets. A proper smtp sender that dutifully respects the specs (the legit ones do) will try again up until a few days later, where the recipient can whitelist the sender. It's all automatic so it kind of doesn't solve OP's issue, but I feel like there should be a proper way to handle the challenge-response part.