By default Zimbra uses DoSFilter Throttling Mechanism to avoid large number of request over a short period of time. So if the Zimbra server see requests more than safe amount from an specific IP address / user that will be throttled and the user may face the “A network service error has occurred” error.
This feature is available in 8.5+ to prevent a malicious IP from trying brute force attacks.
For ZCS 8.7 or above
zmprov mcf zimbraHttpThrottleSafeIPs 10.1.1.2/32 zimbraHttpThrottleSafeIPs 192.168.9.0/24
or to append to an existing list of multi-valued zimbraHttpThrottleSafeIPs
zmprov mcf +zimbraHttpThrottleSafeIPs 10.1.1.2/32
zmprov mcf +zimbraHttpThrottleSafeIPs 192.168.9.0/24
For ZCS 8.6, 8.5 or previous
zmprov mcf zimbraHttpThrottleSafeIPs 10.1.1.2 zimbraHttpThrottleSafeIPs 192.168.9.5
or to append to an existing list of multi-valued zimbraHttpThrottleSafeIPs
zmprov mcf +zimbraHttpThrottleSafeIPs 10.1.1.2
zmprov mcf +zimbraHttpThrottleSafeIPs 10.1.1.50
zmprov mcf +zimbraHttpThrottleSafeIPs 192.168.9.5
zmprov mcf +zimbraHttpThrottleSafeIPs 192.168.9.6
Mailbox server restart is required when modifying these attributes.
zmmailboxdctl restart
Check these links below for more parameters on blocking
https://wiki.zimbra.com/wiki/DoSFilter
https://wiki.zimbra.com/wiki/Users_get_%27Network_service_error%27,_and_mailbox.log_shows_%27Access_to_IP_x.x.x.x_suspended,_for_repeated_failed_login%27