If it's user@domain, the same answer as your email provider ? If it's P2P, basically: everyone by disk space and bandwidth. We might need to invent an easy and

If it's user@domain, the same answer as your email provider ? If it's P2P, basically: everyone by disk space and bandwidth. We might need to invent an easy and secure way to pay first though :) Edit: and a comment here gave me another idea: offline syscs to friends when your PC is online (or syncs from friends)