Email error: Server rejected one or more recipient address

[hb]

Hi,

I am migrate an ASP.Net site from a Windows 2000 Server to a Windows 2003
Server.
But the email function stops working on Windows 2003 Server for this error:
===
The server rejected one or more recipient addresses. The server response
was: 550 unknown user <[email protected]>
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.

Exception Details: System.Runtime.InteropServices.COMException: The server
rejected one or more recipient addresses. The server response was: 550
unknown user <[email protected]>
===

I have followed the steps mentioned by Wil Dobson in his post at:
http://forums.aspfree.com/archive/t-24201

But the error is still there.

The test code is:
MailMessage msg = new MailMessage();

msg.To="(e-mail address removed)";

msg.From="(e-mail address removed)";

msg.Subject="trest";

msg.BodyFormat = MailFormat.Html;

msg.Body = "test";

SmtpMail.SmtpServer="127.0.0.1";//"localhost";

SmtpMail.Send(msg);

msg = null;

Would you please give me some help?

Thank you

HB