Drupal On An IIS Server Running Microsoft Exhchange Getting A Relay Error When Trying To Send E-Mail Notifications

Hi,

I am running Drupal on a Microsoft IIS that is also my Microsoft Exchange server. When Drupal tries to send an e-mail notification (like when someone tries to register, for instance) I get the following error:

details
Type: php
Date: Wednesday, 2006, July 5 - 21:06
User: Anonymous
Location: http://www.mydomain.com/index.php?q=user/register&destination=comment%2F...
Referrer: http://www.mydomain.com/index.php?q=user/register&destination=comment%2F...
Message: mail() [function.mail]: SMTP server response: 550 5.7.1 Unable to relay for new.user@domain.com in c:\Inetpub\wwwroot\modules\user.module on line 410.
Severity: error
Hostname: 63.76.139.12

When it sends e-mail to an account on my Exchange server, however, it works properly with no error in the log, just the notification that an e-mail was sent.

Any help would greatly appreciated.

My environment is as follows:

Server OS: Microsoft Windows Server 2003
Web Server: Microsoft IIS
E-Mail Server: Microsoft Exchange
Drupal Version: 4.7.2
Database: MySQL 5.0
PHP Version: 5.1.4

All of this is on a single server that is also my Domain Controller.

Thank you!

-Pugs