exim4 relay only to specific domain

Hi,

I am new in exim. I am trying to configure a debian server running exim4 such that it relays only mails to our domain (@abcd.de).The exim server uses a smarthost for relaying. I do not get it to block outgoing mails to domains different from ours (i.e. with the "standard" configuration it sends e-mails everywhere).

I would like something like:
if destination domain = abcd.de do relay through the smarthost
if destination domain != abcd.de reject relay

any idea?