[Exim] 2 mail setup questions

Hi,
I have got one internet email, I want to use for the company :
When an email is received from the internet email, I am using a .forward
file to forward the messages to all my local users.This works but I
would like to keep the message in the forwarding user too.


But now, I would like the local users being able to reply to the message
and send a response over the internet. This means the how to translate
automatically usera@local or userb@local into email@??? over the
internet ?

Thanks
Steph