[exim] An appeal for _exact_ directions

Assembled Wisdom!

I have been assured by Chris Siebenmann and Marius(Cyborg)
that what I am after is a cinch in exim.

I postulate a Linux box, with exim, and with three users,
alan, ben, and charles. Let us suppose the name of this
machine is abc.greatbox. Let us also suppose that Alan,
Ben, and Charles have hired a professional mailserver,
him.com, to act as a smart host, or relay host.

I want to know how to configure exim to behave as follows:
if either Alan or Ben or Charles sends mail to simply
alan, or ben, or charles, or root, exim will simply pass
that E-mail on to the addressee. If mail is sent to
'root', the /etc/aliases file will be used to pass the
mail on to whichever of alan, ben, or charles is the
sysadmin. If mail is sent to sophronia, or zephyr,
an error message will be returned: No such person on
this machine.

On the other hand, as soon as a '@' is detected. e.g.
mail to be sent to obama@???, or wherever
else, a diffent part(router?) takes charge, and
the mail is passed on up to the mailserver, him.com,
as coming from alan@???, or ben@???, or
charles@???, as the case may be.

Can some expert take the time to describe _exactly_ how to
do this? Which files, in which of the six directories
acl/ auth/ main/ retry/ rewrite/ router/ transport/
would have to be changed, and how? I would hope for
exact prescription of the syntax; for example, is
foo:bar the same as foo : bar ?

I personally need this _exact_ documentation because I am
an exim newbie; but also because it requires a careful
modification, as root, of important files. And I am
convinced that such a document, giving an exact example,
would be of great help to aspiring exim users, since the
situation I have described cannot be infrequent.

I am asking a lot, I know, but concrete examples of how
exactly to do things are worth their weight in gold.

TIA for anticipated help.

Alan

--
Alan McConnell alan @ razor dot globaltap dot com
"If brute force doesn't work, you are not using enough."