Esther Schindler's article on e-mail for managers is out, and it contains a short paragraph proposed by yours truly:
From the server's point of view, the only thing that matters for routing is the e-mail envelope. This is easily visualized. Grab a sheet of paper, write any name on it (say, "Esther") and fold the page into an "envelope." Write a different name on the envelope (for example, "Sandy"). The envelope will be "routed" through the post office to Sandy, independent of whatever the envelope contains.