[Exim] Intercepting mail from certain addresses on certain h…

Hi folks,

I think someone's making unauthorized use of mail services on one of the
hosts for which my exim-3.36 mail server does relaying.

I'd like to intercept mail from this host when the sender address is
_not_ in a list of about 5 addresses that I know about and delivery
would involve relaying.

I don't mind if this involves a rewrite for every remote address;
duplicates are the least of my worries. :-)

What's the best way to go about this? I'm fluent in exim-3.36
configuration, so I don't need to be spoonfed. I'm just looking for a
general approach, rather than a cookbook excerpt.

Thanks,
Sheldon.