Like it or not, email-without-introduction is mission critical to the Internet.
We just can't break that in general. So incompatible changes to the core
protocols don't make sense now.
Quite right. There have been endless anti-spam proposals that amount
to dividing the world between the good guys inside the perimeter, and
the bad guys outside. But this just replaces the spam problem with
the introduction problem, which is at least as hard.
I've set up a taxonomy of anti-spam techniques in the ASRG wiki:
It's reasonably complete, but if anyone can think of ones that are missing,
drop me a note and I'll give you a password so you can add them.
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