That mail is cheap and efficient is a feature, not a bug. It was designed that way, and the internet cost contract (I pay for my end, you pay for yours) defeated all the other networks, including the pay to send E-mail networks for very good reasons. We want to undo it because we can’t think of anything better for spam?

There are lots of better solutions to spam than this. For example, the system I use is highly efficient and very close to 100% accurate. Namely, take the best available filters. Discard what they are very, very sure is spam. Pass what they are very sure is legit mail. Challenge the in-between with a simple turing test. In practice, the turing tests are very rare and vastly less annoying than having to talk to a secretary to reach somebody. In addition filter only public mailboxes, private addresses never revealed in public need no filters.

Point is, it works. Nobody’s mail is lost as they get the chance to confirm. A little spam can cheat the filters, but it’s nothing to lose sleep over. When there are solutions that work, why would we want to deliberately break E-mail?