[plug] exim question

[John]

I use exim for mail handling (I understand it better than
sendmail/postfix).  I say this because I am asking a question, not
because I want to start a flamewar a la 'my MTA is better than yours...'

What I want to know is, I have several dns entries pointing at my IP,
and users who receive email in each domain.  So, for instance, I have
borg.apana.org.au and john.org.au (not a real name, but...)

Now, I want to be able to send email to auser at borg.apana.org.au and have
it accept, but to auser at john.org.au and have it bounce.  It also needs
to work the other way - anotheruser at borg.apana.org.au bounces, but
anotheruser at john.org.au doesn't.  And there are users who need to get
email in either domain (john at borg.apana.org.au and john at john.org.au). 
Where a username works in both domains, it will always be the _same_
user.

So, my question is, how do I do this?  In exim is preferable, sendmail
or postfix acceptable...

-- 
John <locutus at borg.apana.org.au>
APANA WA Huntingdale MAS