Consider that there are a lot of highly automated email senders that this will never fly for. From small websites/businesses to large (and especially gargantuan

Consider that there are a lot of highly automated email senders that this will never fly for. From small websites/businesses to large (and especially gargantuan!) are more and more likely to contract out email delivery to a specialized provider (e.g. SendGrid, Mandrill/Mailchimp, etc). Even if you focus on transactional emails only (e.g. signup/order verification, password resets, billing notifications, etc) where users are implicitly or explicitly opted in... the amount of mail volume involved is massive. Given that this is a pure overhead charge, you can be sure that email providers are going to pass the cost on to the senders (e.g. whoever owns the relationship with the end-user). The larger that organization is the more likely they'll build that into the cost of the product and pass it down to the user. SPF/DKIM/etc are a huge help. Even when spammers use it correctly it provides reliable attribution for establishing (or rather, destroying) reputation for the domain involved. I would love to see something more and think your intuition is good: an increase to the cost of send email is more likely to weed out illegitimate/unwanted messages.