Perhaps a new signature technology can be used to prove (or at least lend credence to) human authorship? Something like a marriage of a digital signature with

Perhaps a new signature technology can be used to prove (or at least lend credence to) human authorship? Something like a marriage of a digital signature with a captcha: the message has a digital signature of the sender that can be verified with their public key, but it is somehow verifiable that the particular signature provider only does the signature if a human being completes the (difficult AI-proof) captcha. Something like this approach can at least mitigate the mass AI email problem, although the one-off AI emails are unlikely to be slowed by this approach.