See also:
http://en.wikipedia.org/wiki/XMPP#Message_delivery_scenario
On the subject of federation, here's something I posted to my blog about a
month ago; it's a follow up to a comment I had made re the fact that
contact pages on Facebook have no drop-down option for generic XMPP
servers:
I
Hunger wrote:
“Web 2.0″ is that it is a step back in many ways; even AOL — who, of
course, controlled much of Web 1.0 — let you send email to non-AOL users.
Actually, for a long time they didn't. It took a lot of pressure from
their users to allow email in and out of their system.
-Evan
