[Exim] Howto set up an internet email for a group of users ?

Hi,

I have got one internet email only for a group of users, I would like
exim to be able to select the
right outgoing email, depending on the target domain (localdomain or
internet domain)

I would like my users to use :
userxxxxx@localhost when the target is a localhost user , and
internetemail@??? when they send an email to an internet email
account.

Bye
steph