Zimbra webmail showing error" A network service error has occurred"

Since Zimbra v 8.5:

If you see this error "A network service error has occurred" when trying to login to HIS.com Zimbra webmail, your home or office IP address has been blocked or throttled for

• too many logins per IP
• too many failed logins per IP

Stop trying to login and investigate the password problem. If you are sure you have the correct password and the problem persists for more than 30 minutes:

Please check your IP Address from the computer having trouble connecting to our server:

• Open your favorite Browser
• Go to http://www.whatsmyip.org/
• Copy the IP Address shown
• Contact HIS.com Support to report the issue and open a support ticket.

And we will try to unblock it for you.

After you are un-blocked

If you are seeing any error messages from one of the devices you are using to check your email, please fix it ASAP by entering the correct password for your account
or from a device you don't use anymore for checking mail, remove the account from the device Mail App.

If you don't do that, our firewall will detect those repeated failed logins as an attack and your IP address will be blocked again.