Problem with False NDR's on appliance

[genghisosmosis]

Hi Folks
I'm new to this zimbra thing and am having the problem of false NDR's arising from my zimbra server. If i send a mail from an external account to my "work" address, it will be delivered. And I get an NDR from the server :

This is the mail system at host zappa.XXXXX.ie.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<XXXXX@XXXX.ie>: XXXX.ie

Final-Recipient: rfc822; xxxxx@xxxxx.ie
Original-Recipient: rfc822;xxxxx@xxxx.ie
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; xxxxx.ie
Any ideas ?
I've currently got the appliance relaying mail through a clearOS box, and the clearOS box receiving inbound mail and passing it to Zimbra.
I initially had the Zimbra appliance doing it all itself, and it was the same story.
Any help would be greatfully appreciated, please be gentle !
Just to be clear, this isn't a NDR spam issue, mail sent to a legitimate account on our zimbra server is being flagged to the sender as a failure.....

[phoenix]

Just to be clear, this isn't a NDR spam issue, mail sent to a legitimate account on our zimbra server is being flagged to the sender as a failure.....

You will need to look in the log files and see what the exact error is.