[exim] Exim + FreeBSD Open Mail Relay

I am having no end of trouble trying to get Exim 3 to not act as an open
mail relay on FreeBSD. I have set the options "host_accept_relay" and
"host_reject" to allow only internal network connections, and yet when I
connect to the server using the external interface tun0 i can send mail
to whoever i like, from whoever i like. It's as though it's using a
different configuration file, but i know it is using the correct file as
it stops working when i change the address of the mail forwarder.

I have been using Exim3 for years on linux and never had this problem.
Does anybody know whether there are some catches to getting these
options to stick when using FreeBSD?

Regards
Richard