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.