How to bounce emails selectively with exim?

[Oliver Elphick <olly@lfix.co.uk>]

We get a number of spam mails and viruses sent to us with the sender
address spoofed to appear to be from our domain.  These get bounced for
the appropriate reason (unrouteable address, spam, etc) but if the mail
got routed through our ISP, the ISP sends the bounce straight back to me
because they think the sender is me (though the received headers show
the originating machine is in another domain).

For example, here is an unwanted bounce message that has come back to
me; the original message was sent from 203.101.34.73, which claimed to
be lfix.co.uk, but isn't, of course:
========================================================================
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  kevin@lfix.co.uk
    SMTP error from remote mailer after RCPT TO:<kevin@lfix.co.uk>:
    host mailgate.lfix.co.uk [80.177.205.209]: 550 unknown user

------ This is a copy of the message, including all the headers. ------

Return-path: <support@lfix.co.uk>
Received: from [203.101.34.73] (helo=lfix.co.uk)
        by relay-1.mail.demon.net with esmtp id 1DlJws-0002cD-O7
        for kevin@lfix.co.uk; Thu, 23 Jun 2005 05:08:23 +0000
From: support@lfix.co.uk
To: kevin@lfix.co.uk
Subject: Your Account is Suspended For Security Reasons
========================================================================

So if I get a bad email that purports to come from my domain but in fact
doesn't, I would like to delete it without responding at all, while
still bouncing bad emails that were genuinely sent within the domain.

Is there a way to configure exim to do this?

Oliver