MS Exchange 2K3 send and receive mail issue

I recieved the following bounce back mail message when I sent a test message e-mail from my yahoo account to my exchange server. Users on the network can’t internally send e-mail either. message below:

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

hagarshouse@mapcflc.org

Reporting-MTA: dns;srv01.mapcflc.local
Received-From-MTA: dns;web54205.mail.re2.yahoo.com
Arrival-Date: Fri, 11 Jan 2008 10:51:11 -0500

Final-Recipient: rfc822;hagarshouse@mapcflc.org
Action: failed
Status: 5.0.0
X-Display-Name: Tamarah Thompson