[exim] use smarthost depending on initial recipient data?

Hey,

I want to forward certain emails through a smarthost. What makes this a bit
more complicated is that I want to make this depended on the *initial*
recipient address.

One of my problems was coming up with good google keywords so I'm trying to
explain the situation:

A) Exim receives email for foo@???. This email should go through a
   smarthost even if there is a redirect somewhere in the middle (e.g. a
   forward file). So if foo@??? is redirected to baz@???
   it should still go through the smarthost.
B) If Exim receives an email for baz@??? it should try sending it
    directly UNLESS the initial recipient was foo@???.

So my question is if the initial recipient is available in some variable? Or
alternatively could I use one of the built-in redirect options (e.g. .forward)
and somehow "tag" the resulting email/address so I can query that later on in
the routing process?

thanks,
fs