We saw the effects of this recently when suddenly, for no reason, the outbound queue on one of our hosting servers was piling up with messages even though our internet pipe was 100% up. So, I started doing some testing......we had absolutely zero name resolution for remote hosts. At one point, the queue was almost backed up enough to fill up the drive on the server. There were over 3 million outgoing messages that had no way to get out because the remote host names couldn't be resolved, because of this DNS attack.
Then, all of a sudden, out of nowhere, the queue started emptying and the messages started going out. The whole episode lasted about 45 minutes
It was crazy
I had one of my engineers research it and he came up with this article.