Undelivered Mail Returned to Sender

Hi All,
Getting error “Undelivered Mail Returned to Sender”, but the recipient receives the mail though.
Deleted and recreated the user with the same name from Workgroup Manager, issue persists. But works fine with different username
This issue seems to be with 3 users of 800 users
Have tried repairing the permissions, issue still persists

Issue seems to really weird…Any help on this please

Error:
================
hostname.domainname.com. ==> Fully Qualified Domain Name
username@domainname.com ==> Email Address
================
This is the mail system at host hostname.domainname.com. ==> Fully Qualified Domain Name

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

<" "@hostname.domainname.com> (expanded from ): user
   unknown
Reporting-MTA: dns; hostname.domainname.com
X-Postfix-Queue-ID: 2DDF62B798C6
X-Postfix-Sender: rfc822; username@domainname.com
Arrival-Date: Wed,  6 Jan 2010 17:23:41 +0400 (GST)

Final-Recipient: rfc822; ” “@hostname.domainname.com
Original-Recipient: rfc822;username@domainname.com
Action: failed
Status: 5.1.1
Diagnostic-Code: x-unix; user unknown

From: username
Date: January 6, 2010 5:21:07 PM GMT+04:00
To: username
Subject: test mail

5:21