Can someone shoot this thing down: What if we think 'data synchronisation protocol' instead of 'e-mail protocol'? E-mail would then be an 'append only' application of this. The protocols focus would be to register a piece of data, keep track of the 'owner' and sharers and maintain synchronisation (preferably in a peer-to-peer way). I know this problem is big, but once it's done in a reliable way it would be simple to build a good e-mail-style communication on top of it.