Undeliverable: Delivery Status Notification

i use exchange 2000 server as mail server,and another PC with winproxy as communication server , i can send external msges normaly from the clients through the proxy ,but when i forward any msge from the sent items in the microsoft outlook it give me the following msg after one day :

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

Subject:
Sent: 10/2/2004 12:20 PM

The following recipient(s) could not be reached:

mohamed.abnaof@ctc.sd on 10/4/2004 12:27 PM
Could not deliver the message in the time limit specified. Please retry or contact your administrator.

one more thing in the outgoing and incoming mail server i set the ip of the proxy PC