NAV GATEWAY Bounce back problem

Hi all,
I am using NAV gateway for virus protection and mail server is sendmail in linux. But the problem is message won't bounce back to sender if user is unknown. I think There is no problems in sendmail cause message bounce back till NAV gateway and it get stuck there. If i see the log to it shows that bounce mail is 0.
log from NAV

**********************************************
10-Jan-2005 12:21:29 Action: Message Accepted Client: xxx.xxx.xx.xx From: abc@xyz.com
To: helloworld@xyz.com Subject: test Size: 808
SMTP ID: M2005011012212832068

10-Jan-2005 12:21:29 Action: Message Delivery Attempt Failed To: helloworld@xyz.com SMTP ID: M2005011012212832068
Last Response: 550 5.1.1 <helloworld@xyz.com>... User unknown
*********************************************

NEVER GET BOUNCE BACK TO SENDER..


If anyone can help me in this matter. The mail has to bounce saying that user unknown.
Thanks
Regards
Sudadg