gmail handles a lot of mail, which again puts an upper bound on how difficult the puzzle will be. I don't think this changes anything. And if you make it too ha

gmail handles a lot of mail, which again puts an upper bound on how difficult the puzzle will be. I don't think this changes anything. And if you make it too hard, "free" (ad-supported) email will no longer be possible because the costs are simply too high for the financial return.