Some emails are bouncing back with the error message below

The following organization rejected your message: cluster-m.mailcontrol.com

Also when I looked the further details it gives me this information:

Diagnostic information for administrators:

Generating server: myserver.com.au

example.emailadd@domain.com
cluster-m.mailcontrol.com #554 5.7.1 Access denied ##

Received: from myserver.com.au ([192.168.0.3]) by
 myserver.com.au ([192.168.0.3]) with mapi; Mon, 27 Jun 2011
 08:04:50 +0800
From: XYZ <xyz.xyz@mydomain.com>
To: "XYZ (xyz.xyz@externaldomain.com)"
    <xyz.xyz@externaldomain.com>
Date: Mon, 27 Jun 2011 08:04:49 +0800
Subject: FW: Pic S979888
Thread-Topic: Pic S979888
Thread-Index: Acw0WppDIX2PPJwZR0OGVP1rbUtzDAAAzcuA
Message-ID: <573874A6BF36864EA3FB179BF7A43C2B031D388DF7D8@myserver.com.au>
Accept-Language: en-US, en-AU
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
acceptlanguage: en-US, en-AU
Content-Type: multipart/mixed;
    boundary="_004_573874A6BF36864EA3FB179BF7A43C2B031D388DF7D8bunsrvapp00_"
MIME-Version: 1.0

Could you please tell me what is wrong with this and why is it bouncing back?