Get NDR From System Attendant when save calendar items in OWA

i’m using WIN2003 SP1 with Exchange 2003 SP2 and i get this NDR when i save calendar items from OWA

From: test user 1
Sent: Thu 3/8/2007 12:31 AM
To: Microsoft System Attendant
Subject:

Your message did not reach some or all of the intended recipients.

Subject:

Sent:

3/8/2007 12:31 AM

The following recipient(s) could not be reached:

Microsoft System Attendant on 3/8/2007 12:31 AM
The message could not be delivered because the recipient’s destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.

Can someone help me with this issue?

Thanks in Advance..