LinuxQuestions.org - Postfix: why is 'Return-Path' of bounce message empty

I found that in my postfix, the bounced mails that return to senders have 'Return-Path: <>', if the mails come from internet, this make the failures of "returning to sender", because:
"You are not authorized to send mail as <>"
"invalid user - <> SIZE=2509"
"from=<>"

I'm not clear about the meaning of 'Return-Path', is there anyone can give me an explanation?

I explored the mailbox, and found:

Code:

--AE2F8F14083.1135734870/mail.SITE.com--



From MAILER-DAEMON  Wed Dec 28 09:54:39 2005

Return-Path: <>

X-Original-To: USER@SITE.com

Delivered-To: USER@SITE.com

Received: by USER@SITE.com (Postfix)

        id 166EBF14086; Wed, 28 Dec 2005 09:54:39 +0800 (CST)

Date: Wed, 28 Dec 2005 09:54:39 +0800 (CST)

From: MAILER-DAEMON@SITE.com (Mail Delivery System)

Subject: Undelivered Mail Returned to Sender

To: USER@SITE.com

MIME-Version: 1.0

Content-Type: multipart/report; report-type=delivery-status;

        boundary="BB102F14085.1135734879/mail.SITE.com"

Message-Id: <20051228015439.166EBF14086@mail.SITE.com>



This is a MIME-encapsulated message.



--BB102F14085.1135734879/mail.SITE.com

...

This site has a valid DNS name. I don't know why it becomes user <> since there is "From: MAILER-DAEMON@SITE.com (Mail Delivery System)"?

Some mails were bounced by the remote server, and I found that the 'Return-Path' of the message which my server sent is right(USER@SITE.com). Why?

Thank you.