Host or domain name not found

[nnmmss]

I have Mdaemon server and my users can send and receive email without any problem , but sometimes (it is about 3 days) they say that when they want to send email to special domain email address, they got this error
?
eporting-MTA: dns; hdns1.company2.com
X-Postfix-Queue-ID: C4FD320B102
X-Postfix-Sender: rfc822; nnmmss@company1.com
Arrival-Date: Wed, 1 Jun 2005 11:24:50 +0200 (CEST)

Final-Recipient: rfc822; users@company2.com
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; Host or domain name not found. Name service error
for name=hdns2.company2.com.tr type=A: Host not found
)

what is this error message ,and whose problem is it? my mail server or the other side mail server. and if it is our problem( company1) what?could i do??

any information would be appreciated