Why this user always failed to send mail to that user?

This is using MS Exchange 2010 server. Recently, a user told me that his mail to a certain recipient always got the bounced mail as follows:
_____________________________________________
From: Microsoft Outlook
Sent: Tuesday, 8 March, 2011 9:00 AM
To:sender@abc.com
Subject: Undeliverable: Test Mail
 
 
localhost.localdomain rejected your message to the following e-mail addresses:
 
recipientB@xyz.com
 
localhost.localdomain gave this error:
Verification failed for <sender@abc.com> The mail server could not deliver mail to sender@abc.com. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries. Sender verify failed
 
A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk.
 
 Diagnostic information for administrators:
 
Generating server: Exchserver.exchost.local
 
recipient@xyz.com
localhost.localdomain #550-Verification failed for <sender@abc.com> 550-The mail server could not deliver mail to sender@abc.com. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries. 550 Sender verify failed ##
 
Original message headers:
 
Received: from EXCH.exchost.local ([fe80::5015:ecbe:81d4:cf50]) by
 EXCH.exchost.local ([fe80::5015:ecbe:81d4:cf50%11]) with mapi id
 14.01.0218.012; Tue, 8 Mar 2011 09:00:10 +0800
From: sender@abc.com
To: "recipient@xyz.com"
Subject: Test Mail
Thread-Topic: Test Mail
Thread-Index: AcvdLTzKH9kK4f2aShmtXMB6Mk6ZOw==
Date: Tue, 8 Mar 2011 01:00:09 +0000
Message-ID: <DF6FB63728F4494BB9CD9FD2FDB2874C02FA34A0@EXHOST-EXC.exchost.local>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [119.74.141.240]
Content-Type: multipart/related;
        boundary="_004_DF6FB63728F4494BB9CD9FD2FDB2874C02FA34A0EXHOSTEXCexchos_";
        type="multipart/alternative"
MIME-Version: 1.0
 
What's wrong with the sender/recipient's mail? How to resolve it?