[exim] Reject mail destined for subdomain

Hi,

I've got a list of hosts (entire subdomains in fact) that cannot recieve
mail, but send some that is generated by interactive jobs.

Occasionally, mail gets send from these hosts with a wrong destination
address. This causes multiple NDRs from the mailhost, which cannot be
delivered because the sendinghosts cannot receive any mail. This causes
the queue on the mailserver to fill up.

How can I discard all mails to these hosts (subdomains) at the incoming
mailhost?

Thank you,
Ger.