Exim rewrite configuration

[Felix Erkinger <erkinger@gmx.net>]

Hi !

I have a question about doing a rewrite of email-addresses on
a mail server running exim (stable).

Provided by eximconfig, i found a nice little line
which changes addresses from localhost to something different,
by looking up /etc/email-addresses, which is good, with the exception
that i dont want it for mail destinated to other local users

eg. from: felix@mars to: birgit@mars should not be rewritten

i know that exim allows conditional replacements via the if statement
but, no idea how to test the destination address  

Any simple idea (even better, a script snipet) to do this ?

Thanks in advance,
   Felix.