[exim] Block forwarding to certain domains

Hello,

I have a requirement to block forwarded email to certain domains.

If for instance the .forward says something like: "deliver
a.person@???" I need to be able to stop it and return a message to
the sender.

I've set up a list of domains I can search from the userforward: router
but whatever conditions I try in userforward: I can't get it to work

Any help gratefully received.

Vlasta