[exim] bounce non existent users but localdomains

Hi Folks!

I have some domains in localdomains spammers are sending mails to
consisting of postmaster@ or webmaster@ which some domains don't have.

exim4 says: "lowest numbered MX record points to local host".

But how do I let mails bounce to reciepients whose domain _is_ in
localdomains but where no local_part matches?

Kind regards, Konsti

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