[Exim] spammer exploit - using unroutable address

I have seen some emails come in addreassed to:

    [somehex].[somehex]@mydomain.com
with a Return-path: of someone who is a target of spam.  Exim is giving
the error "Unroutable Address" and notifying the email at the
Return-path.  ie: the target of spam.  How can exim be configured to
kick this email back at the sender instead of the faked sender in the
Return-path?

Thanks

running exim4 on debian sarge