On the contrary (as I argued before), rfc-822 mail addresses are
independent of routing! E.g. at our site all outgoing mail from a
workstation is first moved onto the central mail server machine which
takes care of delivery using whatever method it sees fit. In fact
most of our users won't even recognize "smtp" as the name of a mail
delivery protocol.
In general different sites may have different mechanisms to deliver
mail, but the same mail address works for all (ideally, anyway).
Also note that the "host" part of a mail address needn't be the name
of a real host -- it is just a mail domain. The use of
"smtp://host/user" smells too much of direct delivery at a particular
host, while in fact the proper way to send mail is to hand it to a
local agent that knows what's right...
--Guido van Rossum, CWI, Amsterdam <Guido.van.Rossum@cwi.nl>