Exchange NDR 5.5.0 Error

When I tried to reply a message to one of my senders, I received the following:

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

Subject: FW: Undeliverable Mail
Sent: 10/5/2001 4:43 PM

The following recipient(s) could not be reached:

addressee@othercompany.com on 10/5/2001 4:43 PM
Your mail system could not find a way to successfully communicate with the destination system. Please notify your administrator.

I have an Exchange 2000 server.

Thanks.
hongb@visto.com