[Exim] relaying to self-masquerade host?

Dear exim gurus,

I'm trying to setup exim so as to relay mail for an experimental machine
that is hosted on a masqueraded network.

The whole network is masqueraded on the Internet as hell.mine.nu;
the server for hell.mine.nu, Samaris, is
a debian GNU/Linux machine running exim 3.35-1.
I would like it to accept mail for limbo.hell.mine.nu
(the machine actually has many aliases),
with the catch that in the DNS (that also runs on Samaris),
limbo is (rightly) seen as the same IP that Samaris uses
to masquerade the whole network.
On the local network, Samaris is Samaris.porte31 or 10.31.0.1,
and the machine is named limbo.porte31 or 10.31.0.13.

What should I put in my exim.conf so it will work?
What are the relevant sections of the documentation?
What if I want Samaris to accept all for hell.mine.nu,
and only send selected subdomains to limbo?
What if I want Samaris to redirect most mail for hell.mine.nu to limbo,
but still accept Samaris.hell.mine.nu as its own?

Thanks a lot for any help,

[ François-René ÐVB Rideau | Reflection&Cybernethics | http://fare.tunes.org ]
[  TUNES project for a Free Reflective Computing System  | http://tunes.org  ]
Reason isn't about lack of a prejudices, but about presence of postjudices.
        -- Faré