Att rejects emails

We have had a Windows SBS 2008 Exchange Server 2007 for more than a year now running decently. One staff member has been trying lately to send emails to a contact but she keeps getting the rejecting message below. The recipient added us to his safe list as contacts but it did not help. We have more than 10 email accounts set up in the server for our staff. The funny part is that we tried sending an email to the same recipient from another email address from our same domain and it does work; it was not rejected. It rejects only from my coworker email address... please help. This is mining our productivity...




Delivery has failed to these recipients or distribution lists:
OS2583@att.com
An error occurred while trying to deliver this message to the recipient's e-mail address. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message, or provide the following diagnostic text to your system administrator.

The following organization rejected your message: nbfkord-smmi21.seg.att.com.
 _____  
Sent by Microsoft Exchange Server 2007

Diagnostic information for administrators:

Generating server: EDC2K8SERVER.edcdomain.local
OS2583@att.com
nbfkord-smmi21.seg.att.com #554 Denied (Mode: normal) ##
Original message headers:
Received: from EDC2K8SERVER.edcdomain.local ([fe80::33f7:2ce8:4334:2f88]) by
 EDC2K8SERVER.edcdomain.local ([fe80::33f7:2ce8:4334:2f88%10]) with mapi; Tue,
 17 Apr 2012 17:10:55 -0700
From: User <staff.user1@eastcountyedc.org>
To: "OS2583@att.com" <OS2583@att.com>
Disposition-Notification-To: User <staff.user1@eastcountyedc.org>
Return-Receipt-To: <staff.user1@eastcountyedc.org>
Date: Tue, 17 Apr 2012 17:10:53 -0700
Thread-Index: Ac0ZumYXDXNJEkkHS5uN3Xj6LPOC0ADOQXswAAECI1A=
Message-ID: <CF956E3DCBF24E45846CFA4A5780AC8C0208D81416A1@EDC2K8SERVER.edcdomain.local>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
acceptlanguage: en-US
Content-Type: multipart/mixed;      boundary="_004_CF956E3DCBF24E45846CFA4A5780AC8C0208D81416A1EDC2K8SERVE_"
MIME-Version: 1.0