[exim] Smarthost configuration

I have a system running Exim 4.5. I want to avoid having full users on
the system, instead I want a list of usernames (e.g. jms) and email
addresses these correspond to (e.g. jo.shields@???). The system
should accept mail from anything on 192.168.0.* to all users in the
list, or from *.*.*.* to a limited number of users on the list
(support@, admin@). No mail should ever end up being stored locally, it
should simply be forwarded to the appropriate people, via a different
smarthost (smtp.domain.com). All outgoing mail should have its headers
rewritten with an @domain.com address. What's the correct approach for
doing this?

--Jo Shields