In my organization around 2000 users are using a multi server installation setup of ZCS 7.1.3. We observed one thing with zimbra, when ever the DNS response is slow, Zimbra goes down and not responding . That's ok , we can understand.
But Zimbra continues to work slugginsh and Throw java exception error , even after the DNS got recovered. a restart of Zimbra application is required for Zimbra to recover and start working.