[BlueOnyx:26336] Backscatter / user not found bounce

[Colin Jack]

We are having issues with spammers sending thousands of emails to non-existent users on our hosted domains and our BX server then bouncing them as "554 5.5.1 Error: no valid recipients" and our IP consequently getting blacklisted for backscatter. 

Microsoft hate us now – Hotmail etc. and block all email from our subnet! :-/

5210R

Is there any practical way to stop “no valid recipient” email being sent out from the server?

Thanks

Colin