You don't need to "cooperate" on a email system in a way blockchain requires cooperation (with strict following on one protocol). People who need to transfer and store messages will pay for it. Bitcoin makes it easy and cheap. That's it.
People who need to transmit messages will not pay for it, because they can send email for free - in order for a spam-solution to be useful, you need cooperation from a critical mass of recipients that they will not accept or send "old free email" anymore.
