Unable to Sent to a Particular User

A user suddenly stopped being able to send to an email address that she had previously been able to email to.  The name changed from his "name" to his email address in her address book, but the email address seemed to be the same.  She can send to other external email addresses, but this user gives the following errors in order:

Subject: Test Email
 
This message has not yet been delivered. Microsoft Exchange will continue to try delivering the message on your behalf.
 
Delivery of this message will be attempted until 10/2/2011 12:00:27 PM (GMT-05:00) Eastern Time (US & Canada). Microsoft Exchange will notify you if the message can't be delivered by that time.

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Delivery has failed to these recipients or distribution lists:
 
'[emailaddress]@bellsouth.net'
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.
 
  _____  
Sent by Microsoft Exchange Server 2007

 
Diagnostic information for administrators:
 
Generating server: [server].local
 
[emailaddress]@bellsouth.net
#550 4.4.7 QUEUE.Expired; message expired ##
 
Original message headers:
 
Received: from [SERVER].local ([fe80::25a6:8b3c:c593:1142]) by
 [SERVER].local ([fe80::25a6:8b3c:c593:1142%11]) with mapi; Fri,
 30 Sep 2011 12:00:27 -0400
From: <[emailaddress]@DOMAIN.us>
To: [emailaddress]@bellsouth.net
Date: Fri, 30 Sep 2011 12:00:26 -0400
Subject: Test Email
Thread-Topic: Test Email
Thread-Index: Acx/ihHBr9ccS31MSxm24HuhTWRcIg==
Message-ID: <84F9AA08195BA74F9A5E9E2490D23A630500D0B080@[SERVER.local]>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
acceptlanguage: en-US
Content-Type: multipart/alternative;
        boundary="_000_84F9AA08195BA74F9A5E9E2490D23A630500D0B080AFCISERVERafc_"
MIME-Version: 1.0