my first exchange 2000 server

Hi,

I have setup my first exchange 2000 server.
The problem that I am experiencing is that when someone sends a email from ea hotmail.com they will receive a return receipt:

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

username@domain.com

Reporting-MTA: dns;brion-dc01.BRION.local
Received-From-MTA: dns;vici.pancarib.com
Arrival-Date: Thu, 21 Mar 2002 12:37:30 -0400

Final-Recipient: rfc822;username@domain.com
Action: failed
Status: 5.3.5

What am I missing ??

Regards,

Leopold