Most bigger services are probably going to send and receive about the same amount of mail so it will be an almost zero-sum game. Most services today have a cap

Most bigger services are probably going to send and receive about the same amount of mail so it will be an almost zero-sum game. Most services today have a cap on how mail you can send, and this could be made more explicit and have cases where the end user actually pay. I don't really see the payment as a big problem. This has been discussed previously, mostly initiated by the big email services