HELP ME : Illegal host/domain name found

Hi,

One of my customer can not received some email from outside. The reason is because Illegal host/domain name found, like describe on email header below :

========================
Sender : Internet Mail Delivery<postmaster@mailout2.domain.com>
Date : Mar 05, 2004 09:13
Title : Delivery Notification: Delivery has failed
This report relates to a message you sent with the following header fields:

Return-path: <denny.felix@domain.com>
Return-path: <denny.felix@domain.com>
Received: from tcp-daemon.mailout2.domain.com by mailout2.domain.com
(iPlanet Messaging Server 5.2 HotFix 1.17 (built Jun 23 2003))
id <0HU200C1CTYOX4@mailout2.domain.com>
(original mail from denny.felix@domain.com); Fri,
5 Mar 2004 09:13:36 +0900 (KST)
Received: from custom-daemon.mailout2.domain.com by mailout2.domain.com
(iPlanet Messaging Server 5.2 HotFix 1.17 (built Jun 23 2003))
id <0HU200CFRTXQQS@mailout2.domain.com> for andi@myclient.com; Fri,
05 Mar 2004 09:13:02 +0900 (KST)
Received: from ep_ms2_bk (mailout2.domain.com [xxx.254.224.25])
by mailout2.domain.com
(iPlanet Messaging Server 5.2 HotFix 1.17 (built Jun 23 2003))
with ESMTP id <0HU2007DXTWNKD@mailout2.domain.com> for andi@myclient.com;
Fri,
05 Mar 2004 09:12:23 +0900 (KST)
Received: from ep_spt01 (ms2.domain.com [xxx.254.225.111])
by ms2.domain.com (iPlanet Messaging Server 5.2 HotFix 1.17 (built Jun
23
2003)) with ESMTP id <0HU200MUPTWN69@ms2.domain.com> for andi@myclient.com;
Fri,
05 Mar 2004 09:12:23 +0900 (KST)
Content-return: prohibited
Date: Fri, 05 Mar 2004 00:12:33 +0000 (GMT)
From: DENNY FELIX <denny.felix@domain.com>
Subject:
X-Sender: =?windows-1252?B?U0FNU1VORyBFTEVDVFJPLU1FQ0hBTklDUyBJ?=
=?windows-1252?B?TkRPTkVTSUE/U0lYIFNJR01BIFBBUlQ/U0Uy?=
To: andi@myclient.com
Reply-to: denny.felix@domain.com
Message-id: <0HU200MUQTWN69@ms2.domain.com>
MIME-version: 1.0
Content-type: text/html; charset=windows-1252
Content-transfer-encoding: 7BIT
X-Priority: 3
Msgkey: 20040305001222901@denny.felix
X-MTR: 20040305001222901@denny.felix
X-EPLocale: en_US.windows-1252
X-EPWebmail-Msg-Type: personal
X-EPWebmail-Reply-Demand: 0
X-Generator: NamoMIME 1.1.0.14

Your message cannot be delivered to the following recipients:

Recipient address: andi@myclient.com
Reason: Illegal host/domain name found


Original-envelope-id: 0HU200CFSTXQQS@mailout2.domain.com
Reporting-MTA: dns;mailout2.domain.com (tcp-daemon)

Original-recipient: rfc822;andi@myclient.com
Final-recipient: rfc822;andi@myclient.com
Action: failed
Status: 5.4.4 (Illegal host/domain name found)
=========================

Please help me why this is happening and how to resolve it ?

Thanks