if the /etc/resolv.conf is changed to

nameserver 127.0.0.1
search domainname.local
nameserver 192.168.0.1

The local dns caching server is activated (127.0.0.1) - Zimbra changes this and use the configs:

[zimbra@mail root]$ zmprov getServer `zmhostname` | grep DNSMasterIP
zimbraDNSMasterIP: 192.168.0.100
zimbraDNSMasterIP: 192.168.0.1

To remove a unused server (.100)

 zmprov ms `zmhostname` -zimbraDNSMasterIP  192.168.0.100

or add a new server

 zmprov ms `zmhostname` +zimbraDNSMasterIP  192.168.0.1

Link: https://wiki.zimbra.com/wiki/DNS_caching_service_(dnscache)