Undelivered Emails

I’m trying to send email and i get the response,
“Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 3/26/2007 10:32 AM

The following recipient(s) could not be reached:

‘g.barbanel@att.net’ on 3/26/2007 10:32 AM
There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.
”

Can someone explain this to me? I use exchange server 2000/Outlook 2000. I can’t send mail onlu to this address.