Email not sent to a distribution list

Hi - I am trying to send an email from Mingle to a D=distribution list to keep upper management in loop for some changes. But it is not working. Below is the message i receive. Please help.

 

Delivery has failed to these recipients or distribution lists:

 

ITSyncUp@orhp.com
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.

 

  _____  

Sent by Microsoft Exchange Server 2007 

 

Diagnostic information for administrators:

 

Generating server: ORHPEXCA1.corp.orhp.com

 

ITSyncUp@orhp.com
#550 5.7.1 RESOLVER.RST.AuthRequired; authentication required ##

 

Original message headers:

 

Received: from (172.17.50.77) by orhpexca1.corp.orhp.com (172.17.50.12) with
 Microsoft SMTP Server id 8.2.255.0; Mon, 19 Dec 2011 16:23:11 -0800
Date: Mon, 19 Dec 2011 16:23:10 -0800
From: Mingle <roohia@orhp.com>
To: <ITSyncUp@orhp.com>
Message-ID: <4eefd56e6ab08_1a90be8240e@ROOHI-ACS.tmail>
Subject: Story #1168 CC - Invoicing Tab_Allow access to Invoice Payment History changed by Kathy
MIME-Version: 1.0
Content-Type: text/html; charset="utf-8"
Return-Path: roohia@orhp.com