[Exim] How to change the address when sent externally

Ok, here's the situation I have: I have a box with fetchmail, exim, procmail
etc. to get the mail of external servers and deliver it to users on my LAN. I
called this box ackee. Now I have a bunch of addresses in the form of
user@ackee so that users in the office can send email to each other. That
works fine. What I want to do now is somehow have it so that when a user
sends a message to an outside address, the sender user@ackee will be replaced
by company@???. Is this possible?
I tried looking through the docs but couldn't find this anywhere. If it is
there then go ahead and flame me, just point me to were this is explained.