Spam exists because sending as others here have mentioned is free. The knee-jerk reaction might be to make it not free and technically that would be rather sim

Spam exists because sending as others here have mentioned is free. The knee-jerk reaction might be to make it not free and technically that would be rather simple. Beyond adding pre-approved DKIM signed domains to an approve-list, one could modify existing grey-list daemons to require either a one-time code or click a URL to enter authentication to approve the sender and add rate limiting for know abusers, but that isn't the problem. People are accustom to how email currently works and adding any barriers or changing communication flows or adding cost adds friction. Businesses and people will oppose friction. I would be curious to see if anyone here has added such barriers and what their results were. What methods did you use to make spam expensive for spammers and how did it affect your legit customers and prospects?