WatchGuard SMTP Proxy (None) Deny Message

We have a WatchGuard Firebox 750e running Fireware 10.1 with WSM 10.1.
For some reason the SMTP Proxy is returning a Deny message for only a couple of email domains that are sending system auto-generated email. Other system generated emails are OK.

The user receives the following deny message:
"The FHS Administrator that protects your network has detected a message that may not be safe.
Cause : The message content may not be safe.
Content type : text
File name    :       (none)
Virus status : No information.
Action  : The Firebox has deleted (none).
Your network administrator can not restore this attachment."

Example of a email internet header info is:
Microsoft Mail Internet Headers Version 2.0
Received: from imagemail.asic.gov.au ([61.88.96.2]) by _ with Microsoft SMTPSVC(6.0.3790.3959);
       Tue, 1 Jul 2008 16:13:45 +0800
Received: (from ascqmgr@localhost)
      by imagemail.asic.gov.au (8.11.7+Sun/8.11.6) id m618Dkk26457
      for gjones@firsthomeshop.com; Tue, 1 Jul 2008 18:13:46 +1000 (EST)
Date: Tue, 1 Jul 2008 18:13:46 +1000 (EST)
From: Docimage Application User <ascqmgr@imagemail.asic.gov.au>
Message-Id: <200807010813.m618Dkk26457@imagemail.asic.gov.au>
To: gjones@firsthomeshop.com
Subject: DocImage Request 6668054 [SEC=UNCLASSIFIED]
Content-Type: text/plain; name="message.txt"; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
Return-Path: ascqmgr@imagemail.asic.gov.au
X-OriginalArrivalTime: 01 Jul 2008 08:13:45.0627 (UTC) FILETIME=[61E52EB0:01C8DB52]

I have set a File Name rule to allow (none) but it has not helped. What do I need to configure to stop these emails being denied by the Firebox?
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