[Exim] exim config question

Hi,

I am new to exim and have the following situation. There's a computer on our
network which is used for production and testing purposes, and uses exim to
send email. However, since it's only used for testing purposes, I don't want
exim to actually send email to the outside world. I used to do this by

send_to_gateway:
     driver = manualroute
     domains = !+local_domains
     transport = remote_smtp
     route_list = * nodomain.bzo.domain

which results in nothing being sent since the domain in 'route_list' is
non-existent. The messages become frozen and after 7 days or so they
disappear.
However, as of today, there's one domain I want exim to actually do send
email to. Is there a way to stop exim from sending email, except for a few
domains I want exim to send email to?

Thanks,

Martin