why I receive NDR?

recently one senders from outside sent one email to one of my users  in my server (our server is exchange 2007 and sourece server is exchange 2003 ) he receive following NDR :
do you know why he receive this NDR ?

Your message did not reach some or all of the intended recipients.
      Subject:  FW:
      Sent:     1387/11/19 04:45 (.8
The following recipient(s) cannot be reached:
      myuser@domain1.com on 1387/11/19 04:46 (.8
            A critical function required for the transfer or delivery of the message was not supported by the originator of the report
            <Mail.domain2.com #5.3.3 smtp;500 5.3.3 Unrecognized command>

Who is Participating?
MapnaturbineAuthor Commented:
what you mean ? what can I do to solve the problem ?

also some time when I send email from my domain I receive following NDR :

Delivery has failed to these recipients or distribution lists:

The recipient's e-mail system can't process this message. Microsoft Exchange will not try to redeliver this e-mail message for you. Please provide the following diagnostic text to your system administrator.
Diagnostic information for administrators:
Generating server: mail.domain2.com
#< #5.3.3 smtp;500 5.3.3 Unrecognized command> #SMTP#

Original message headers:

Received: from mail.domain1.com ([]) by mail.domain2.com
 with Microsoft SMTPSVC(6.0.3790.3959);       Sun, 8 Feb 2009 12:50:16 +0330
Received: from srv-htcas1.domain1.com ( by
 smtp1.mapnaturbine.com ( with Microsoft SMTP Server (TLS) id
 8.1.336.0; Sun, 8 Feb 2009 12:50:29 +0330
Received: from domain1.com ( by srv-htcas1.domain1.com
 ( with Microsoft SMTP Server id 8.1.336.0; Sun, 8 Feb 2009
 12:50:00 +0330
From: <user@domain1.com>
To: <user@domain2.com>
X-Mailer: @(#) mailsend v1.06 (Windows)
X-Copyright: GNU GPL. It is illegal to use this software for Spamming
MIME-Version: 1.0
Content-Type: text/plain
Message-ID: <5755db5f-45ec-449a-bbdf-508d2256de93@srv-htcas1.domain1.com>
Return-Path: user@domain1.com
Date: Sun, 8 Feb 2009 12:50:00 +0330
Content-Transfer-Encoding: quoted-printable
X-OriginalArrivalTime: 08 Feb 2009 09:20:16.0236 (UTC) FILETIME=[74304AC0:01C989CE]

two things

1. What account are you using for login account for your exchange services? it should not be domain account but network service account

MapnaturbineAuthor Commented:
we do not have problem ! one user from another company that wants send mail to my user have problem , in fact he is a domain user and his mail sent from MS Outlook .
I understand ..however this is most probably because the configuration issues at E2K7 side.
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