Remote Server returned '554 5.4.4 SMTPSEND.DNS.InvalidData; DNS returned an invalid response for this domain'

Hi guys,

We had a fresh Exchange Server 2013 install. It had been working fine for a day, then all in a sudden, the outgoing emails did not go out and stuck in queue. From the NDR, it says:

Remote Server returned '554 5.4.4 SMTPSEND.DNS.InvalidData; DNS returned an invalid response for this domain'

From the Queue viewer, the last error shows:

I am pulling my hair off to try to fix this problem. Any advise and ideas would be much appreciated.

Henry