Error 451 4.4.0 & 400 4.4.7 only in gmail.com

Hi....

We are using MS Exchange Server 2013  on MS Windows Server 2012 Standard. 

In queue viewer, I am getting this error for gmail.com ( 451 4.4.0 DNS Query Failed.......)

And if we look at the message , we got the following error(400 4.4.7 Message Delayed) :

It happens only in gmail.com. 

Can someone please help to fix this..?

Thanks in Advance..

Regards,

Hilda