Most people don't realize it but email can be routed just like network
packets. If one publishes their email address widely it doesn't make
sense to have to change it when you move your office. That's where
email routing comes in.
I would guess that large organizations with thousands of people find
this a nightmare. It's also something not addressed by mail systems
such as Microsoft Exchange or Lotus notes. This e-mail routing is
really a layer above (or below, depending on your vantage point).
It's between the final work-group email server and the
router/firewall.