[newbee] permit only local mail-delivery to some users

Hi all,

I'm new to exim, and I'm trying to configure it such, that SOME users
are able to send mails to local users and to the internet, and that SOME
users are only able to send mails to local accounts NOT to the internet.
In case the latter sends a mail to a non-local address, he should
receive an error message.

Please, could someone give me help or point me to further information
resource (I've currently reading the exim-docu... to be honest, it's
quite complex for a "newbee").

Thanks, Joe