[Exim] Limiting sending and receiving mail with exim

Hello,

I've been trying to make some limitations concerning receiving and sending
out mail on my domain.

The problem is as follows:

I have a bunch of user accounts on my server. A part of them have rights
to send and receive mail outside my domain. The rest of them are allowed
to send and receive mail inside my domain, but not outside.

What would be the easiest way to set this up? What should I put in my
config files? I think I will use dbm files in my setup.

    Regards,
        Petri