[EXIM] routing-dependend sender rewrite

I've a small intranet using private address space and a private
domain name inside and one of the hosts connected to the internet
with a different domain name.

I need to do sender address rewriting depending on the destination of the
mail, inside the private domain name should be used but when mail leaves
the intranet a different sender address should be used.

Any pointer to a solution is welcome.

Regards, 
         Frank Elsner

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