One could add a proof of permission model to that to cover things like mailing lists and other regular sources of email. Something like:

I sign up for a mailing list.

The mailing list mail server contacts my mail server.

My mail server presents to me a request for free delivery.

I approve it.

My mail server gives the mailing list server a secret token.

Each mailing list message includes proof that the message has been blessed by the sender and that the sending server knows the token.

I think the fee could be applied only to unsolicited emails. If I have a particular email address whitelisted then it will get through my gateway with no fee attached. However, if it is an unsolicited email from someone not on my whitelist it will be required to have some small fee attached to get past my filter.

I've seen some suggestions that instead of using money you could use some proof-of-work computation. This is something we could scale the difficulty factor of as computing power increased. Something that takes, say, an average computer 30 seconds (or longer) to calculate. It will attach this proof-of-work to the email.

Mailing lists wouldn't need to, or couldn't exist in this new magical email system. There's a hundred other solutions out there for what mailing lists provide.

Impose the fee only on one-way communications. A responsible mailing list would have a single instance of two-way communication when asking the user to confirm that they wanted to sign up. No I have no idea how that much state should be handled in a distributed fashion.