I think it's a different problem - mail delivery requires a series of entities interacting with each others (think about a network of mailmen). Also, private m

I think it's a different problem - mail delivery requires a series of entities interacting with each others (think about a network of mailmen). Also, private mail exists and it's definitely useful. Digital messages are usually built on top of the internet (which would have similar problems) and every service can be used across the entire network.