Spam bring returned as NDR's

Hiya,

We own the domain inboundsolutions.co.uk and all mail points to our exchage server in house.
It seems like some spammers are sending mail FROM user@inboundsolutions.co.uk and when the recepient doesnt exist , the NDR's come back to us.

Example.

Your message did not reach some or all of the intended recipients.

      Subject:      Rolex
      Sent:      04/04/2008 18:33

The following recipient(s) cannot be reached:

      lpuige@gandia.org on 05/04/2008 21:24
            The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            < iriscom-atm1.gandia.org #4.0.0 X-Postfix; host iriscom.gandia.org[80.24.166.219] said: 450    4.1.1 <lpuige@gandia.org>: Recipient address rejected: User unknown in    virtual mailbox table (in reply to RCPT TO command)>


Is there any simple way to fix this ? we are already running GFI which gets most spam, but as these are NDRs and we dont want to turn them off...
obviously someone is faking their sender FROM address...

Many thanks,

Shaun Okeefe
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Your message did not reach some or all of the intended recipients.
 
      Subject:	Rolex
      Sent:	04/04/2008 18:33
 
The following recipient(s) cannot be reached:
 
      lpuige@gandia.org on 05/04/2008 21:24
            The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            < iriscom-atm1.gandia.org #4.0.0 X-Postfix; host iriscom.gandia.org[80.24.166.219] said: 450    4.1.1 <lpuige@gandia.org>: Recipient address rejected: User unknown in    virtual mailbox table (in reply to RCPT TO command)>