SMTP Communication Problem!!!!!

Recently, I try to send email to one of my client
through Exchange server 2000 using Ms Outlook XP,but unsuccessfully to send it.Below is the error message that bounce back.

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

Subject: testing email from IT dept(eastin)
Sent: 27/02/2004 5:45 PM

The following recipient(s) could not be reached:

angelia.ku@renaissance-kul.com on 27/02/2004 5:45 PM
There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.
>

if anyone can fix this problem for me??