The thing to notice about this system is that only people who send more emails than they receive lose out. It would end spam at a stroke.

And it doesn't matter if online purchases are more expensive, because you get that money back through email receipt fees.

Tougher challenges are the traditional ones with micropayments. Transaction costs. And maybe tax implications. And the differential incentive based on wealth. People struggling for cash would still try and minimise their outgoing communication, which is probably a bad thing for a healthy society.