Intermittent Email Bounce back on Exchange 2010

Hi

Im just testing exchange 2010 but having a slight issue:

when sending emails to the domain the sender is receiving intermittent bounce backs. I would say 50% of emails are going through fine but the other half are bouncing back with the following error.

Delivery has failed to these recipients or distribution lists:

RRiaz@debt-specialists.co.uk
A problem occurred during the delivery of this message. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message later, or provide the following diagnostic text to your system administrator.







Diagnostic information for administrators:

Generating server: smtp52.relay.iad1a.emailsrvr.com

RRiaz@debt-specialists.co.uk
#< #5.4.4 X-SMTP-Server; Host or domain name not found. Name service error for name=mail.debt-specialists.co.uk type=A: Host not found> #SMTP#

Original message headers:

Return-Path: <rashid.riaz@thedebtspecialists.co.uk>
Received: from smtp52.relay.iad1a.emailsrvr.com (localhost.localdomain
 [127.0.0.1])      by smtp52.relay.iad1a.emailsrvr.com (SMTP Server) with ESMTP id
 9EB8024079E      for <RRiaz@debt-specialists.co.uk>; Thu, 12 Apr 2012 09:49:42
 -0400 (EDT)
X-SMTPDoctor-Processed: csmtpprox 2.7.4
Received: from localhost (localhost.localdomain [127.0.0.1])
      by smtp52.relay.iad1a.emailsrvr.com (SMTP Server) with ESMTP id 99BF22404C2
      for <RRiaz@debt-specialists.co.uk>; Thu, 12 Apr 2012 09:49:42 -0400 (EDT)
X-Virus-Scanned: OK
Received: from smtp192.mex02.mlsrvr.com (smtp192.mex02.mlsrvr.com [204.232.137.43])
      by smtp52.relay.iad1a.emailsrvr.com (SMTP Server) with ESMTPS id 6DCAB24079E
      for <RRiaz@debt-specialists.co.uk>; Thu, 12 Apr 2012 09:49:42 -0400 (EDT)
Received: from IAD2MBX16.mex02.mlsrvr.com ([172.23.11.121]) by
 IAD2HUB06.mex02.mlsrvr.com ([172.23.10.70]) with mapi; Thu, 12 Apr 2012
 09:49:32 -0400
From: Rashid Riaz <rashid.riaz@thedebtspecialists.co.uk>
To: Rashid Riaz <RRiaz@debt-specialists.co.uk>
Date: Thu, 12 Apr 2012 09:49:18 -0400
Subject: hello
Thread-Topic: hello
Thread-Index: Ac0Ysw6bknf6dGDCR067rYvq2tc5ZA==
Message-ID: <0B4490E3C65A1845AC5EB5A83D45FCECD597A606D9@IAD2MBX16.mex02.mlsrvr.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
acceptlanguage: en-US
x-tm-as-product-ver: SMEX-10.0.0.4152-6.800.1017-18676.003
x-tm-as-result: No--11.330600-8.000000-31
x-tm-as-user-approved-sender: No
x-tm-as-user-blocked-sender: No
Content-Type: multipart/alternative;
      boundary="_000_0B4490E3C65A1845AC5EB5A83D45FCECD597A606D9IAD2MBX16mex0_"
MIME-Version: 1.0