My_Server.*********.local rejected your message to the following e-mail addresses

Every time I try to send am e-mail from a client PC to a certain e-mail address running on my exchange server (MS Exchange 2010 Entrprs) I get the following rejection message.... Any help please to work it out?

Thank you,
Costas.


Rejection message that I get:

My_Server.*********.local rejected your message to the following e-mail addresses:
Chrys@**********.com
My_Server.*********.local gave this error:
Client was not authenticated
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.

Diagnostic information for administrators:
Generating server: My_Server.*********.local
Chrys@*********.com
My_Server.*********.local #530 5.7.1 Client was not authenticated ##
Original message headers:
Received: from ChrysanthyPC (192.168.8.109) by mail.*********.com
 (192.168.1.143) with Microsoft SMTP Server id 14.1.****.1; Sat, 11 Aug 2012
 18:40:33 +0300
Return-Receipt-To: Chrysanthy ********<chrys@*********.com>
From: Chrysanthy ********<chrys@********.com>
To: <Chrys@***22222*******.com>
Subject: 777
Date: Sat, 11 Aug 2012 17:40:35 +0300
Message-ID: <!&!AAAAAAAAAAAYAAAAAAAAAPgokavafWFHrlShbnmW2S3CgAAAEAAAAJWlLauQfQ1HkI2ZCaIlIPcBAAAAAA==@**********.com>
MIME-Version: 1.0
Content-Type: multipart/related;
      boundary="----=_NextPart_000_0034_01CD77E8.6AC62160"
X-Priority: 1 (Highest)
X-MSMail-Priority: High
X-Mailer: Microsoft Office Outlook 12.0
Importance: High
Thread-Index: Ac13zzIXyY4rG7rnSIWcTA87bIHvLw==
Content-Language: en-us
Disposition-Notification-To: Chrysanthy *******<chrys@*******.com>
Return-Path: chrys@********.com