[Exim] Mail forwading questions

I've got a couple of questions...

1) How do I forward all incoming mail for a specific domain that is not
routed via the aliases file to a mail server behind a firewall. The 'behind
the firewall' server obviously will not be listed in the MX records. This
front-end Exim server filters all mail, redirects mail with aliases, and
then needs to send the remaining non-local mail to the remote host using the
same domain name.

2) Different box, unrelated application... I have an Exim mail server that
receives all mail for our old company name, filters all mail, resolves the
group emails, and redirects via the aliases file to the new company names
remote mail server. This is working except for problems in the front end
mail server for the new company names host... I know long story, but what I
need to do now is redirect this mail to an IP address so I can deliver it
directly to the internal (behind a firewall) mail server. How do I do this?
Example; mail directed to John Doe in the aliases file now looks like this:

jdoe@???: john.doe@???

Is there a way to direct all mail for newcompany.com to an IP address
without doing a DNS lookup? And to complicate things further, what if I
needed to send selected 'newcompany.com' users to the internal IP address
and others go through the DNS lookup process? (I've been told I may have to
do this, since some users internal mail server for our location is not the
same so will need to be directed at a higher level, or selectively deliver
to a different internal server)

Thanks for your time,


-Craig