[exim] howto force dns-lookup for local domains

Hello,

we allow our users to configure forwardings and they can create accounts
for domains, that aren't registered through us. A possible worst case
scenario would be, that a user creates a *@hotmail.com account
(catch-all) on our mailserver and suddenly all forwardings other users
configured to real external hotmail-mailboxes would end in the local
hotmail-account of a fraudulent user. How can I prevent this? The only
way I can imagine is to force exim to do dns-lookups for all
forwardings, no matter if the domain is local or remote. Any ideas, how
I could configure this?

Regards
Marten