Not necessarily. You could just use a service that is secure and handles the payment aspect for you. In such a scenario the top email providers would send email

Not necessarily. You could just use a service that is secure and handles the payment aspect for you. In such a scenario the top email providers would send email between themselves using the same pool of bitcoin. It's essentially a zero-sum transaction for them at scale. Outside entities that want their message heard would have to pay for it (mailing lists and the like) as a disincentive to send unwanted email.