Well, one can dream :-)
Micropayments may be impractical, but email as it is currently used is also impractical for personal use cases. Outside of the tech world, how many people do you know how primarily communicate via email?
Most have switched to texting, or to closed systems (WhatsApp, etc). Some of these closed systems make it easy for you to block others, and also can tightly control spam.
> First, require all messages to include an `Automated: 0|1` header; eventually downranking / banning providers that don't include it
How do you handle bad actors who set it to 0?
> It's impractical to require pre-approvals on every message, because many people genuinely want human-to-human contact from semi-strangers.
You pre-approve the person, not the message. Once approved, no further approvals are needed.
And human-to-human contact, for me, means more or less 1:1 (or at most 5:1). If a semistranger wants to talk to me, by all means, I'll approve him. What's the concern?
> On the other hand, requiring consent without verification is ineffective, because it is too easy to pretend consent.
Right now I do it by whitelisting the email address. Obviously, it's fragile. In practice, it's never a problem. The only failures are when I get spam with a From address that is mine. No company ever impersonates a friend's email address to get to me.
(Not saying your solution is unworkable, but would need more details).