[Exim] exim to work without restriction to class C IP range

Hi,
I installed exim using debian package.
I need to configure exim.conf to allow
to send emails using outgoing smtp,
but just for our subnet Class C. Nobody
else is supposed to be able to send through
mydomain.com. I have tried a few
options but no success.
Anybody can help me with the needed
directives to restrict everybody, but
my network C Class?

Thanks,
Slava.