Inbound Mail Problem

I can send e-mail with no problem, but unable to receive internet e-mail, using exchange server 2003.

.net 007: This e-mail message was undeliverable due to the following reason:

.net 018: Each of the following recipients were rejected by a destination mail system.

Note: The reasons given by the destination mail system are included to help you determine why each recipient was rejected.

Solution:
Attempt to resend, or contact the recipient by alternate means to let them know about the issue.

Recipient:
Reason: 5.1.1 … User unknown

Reporting-MTA: dns; imf22aec.mail.bellsouth.net
Arrival-Date: Thu, 12 Jan 2006 23:12:10 -0500
Received-From-MTA: dns; ibm65aec.bellsouth.net (65.12.201.176)

Final-Recipient: RFC822;
Action: failed
Status: 5.1.1
Remote-MTA: dns; sgecpa.com (65.83.225.189)
Diagnostic-Code: smtp; 550 5.1.1 … User unknown