[Exim] Help with conditional header re-writing

Hello,

I want to set up a single mail hub to manage internal (local) e-mail,
under the domain "internal.domain," as well as external e-mail. These
are the rules I'm trying to implement:

1. [internal mail] Mail from someone@??? to
someone@??? is not touched.

2. [out-bound mail] Mail from someone@??? to
someone@??? is re-written as sent FROM someone@mydomain.

3. [in-bound mail] Mail from someone@??? to
someone@mydomain is re-written as sent TO someone@???.

Another way to look at this is that:

1. Local users will always see their own addresses as being part of the
@internal.domain.

2. External correspondents always receive e-mail as sent from @mydomain.

The nested ${if .... } I've tried so far don't seem to work. I bet
there's a simple way to do this!

I would appreciate any suggestions.

Thank you,

--
Pedro