Delivery delayed when sending to specific address

[Yassin]

I usually receive the following message when sending to
any adressee at ((@ties.itu.int)):
((Reporting-MTA: dns;unsaexch02.un.org.sa
Final-Recipient: rfc822;[email protected]
Action: delayed
Status: 4.1.8
Diagnostic-Code: smtp;451 4.1.8 Domain of sender address
(e-mail address removed) does not resolve
Will-Retry-Until: Thu, 17 Jul 2003 15:31:21 +0300))

Moreover, I can't receive from senders with @ties.itu.int
on their address.
Solution, please.