It's worth pointing out, though, that having per-recipient prices would require a change to the protocols to allow the sender to know in advance how much it wil

It's worth pointing out, though, that having per-recipient prices would require a change to the protocols to allow the sender to know in advance how much it will cost for them to have their email delivered (and thus whether it is worth them writing the email in the first place), which breaks the "store and forward" architecture of email. Not necessarily...people are already writing emails on spec because there is no guarantee that I will read it. It's just that they send it to thousands of people hoping that the fraction who read it will make it worth the time spent writing the email. Without changing the architecture of email you could reject emails (e.g. 550: rejected, unsolicited emails require $0.50 payment). Or you could passively ignore emails that don't meet criteria (you could piggyback on the read receipt functionality to provide confirmation). All of this is backwards compatible with existing email