> Spam would still be hard, or arguably even harder

Doesn't make any difference. The major sources of spam today are places like gmail and outlook servers. You have to run some message classification regardless of where it came from.

> The major sources of spam today are places like gmail and outlook servers.

By volume perhaps, but is that true as a proportion of all mail that was sent via these services? I'd be surprised, though I'd be very interested if there where any numbers publicly available.

> By volume perhaps, but is that true as a proportion of all mail that was sent via these services?

Would you care? I can say that I don't care (as someone running my own email infrastructure).

I run filtering on all incoming email, I don't assign any weight to which email server it came from because it's not a useful factor.

The spam situation could also be better if we had designed into that space. Want to send a single email to a sender that you don't have an existing relationship with? Hashcash. Have a web service that sends lots of emails? Get user consent beforehand with e.g. an oauth token to bypass or lower hashcash difficulty.