... XMPP is a federated, distributed system I'm surprised nobody in this thread has mentioned SMTP. It's even more widely used than XMPP, and it's a great exam

... XMPP is a federated, distributed system I'm surprised nobody in this thread has mentioned SMTP. It's even more widely used than XMPP, and it's a great example of a federated system (as well as the big issue one has to deal with with open, anonymous federated systems: abuse/spam). Setting up a SMTP (or XMPP) server is not easy, but no "average user" is going to do that. They'll use their ISP, or Gmail, or their work e-mail.