Reverse DNS Disappear

We deploy the Exchange server in Windows Azure, Singapore, and set the reverse DNS lookup, everything was all right, until last week, suddenly found that we reverse DNS disappeared, but we see in the server, record still exist.

We try to delete the original records, reset the reverse DNS, there is still no success.

Who can tell me, what happened, or how should solve?

C:\>nslookup mail.cxxralliance.asia
Server:  UnKnown
Address:  192.168.0.254

Authority reply::
Name:    mail.cxxralliance.asia
Address:  111.221.81.181


C:\>nslookup 111.221.81.181
Server:  UnKnown
Address:  192.168.0.254

*** UnKnown can't find 111.221.81.181: Non-existent domain