See also Troubleshooting Outlook
Outlook can connect to a Zimbra server in three different ways:
When working with Outlook accounts, you can use the Control Panel applet Mail to manage Outlook profiles and accounts.
Newer versions of Outlook (2013 & 2016) support the ActiveSync protocol.
Unlike other protocols, ActiveSync can synchronize mail, contacts, calendars and tasks.
Sometimes it is best to create a new Outlook Profile, then add an account to the new profile.
From Windows Control Panel → Mail:
From within Outlook:
Microsoft Outlook support for the standard IMAP protocol is much improved in newer versions (Outlook 2013, Outlook 2016 and newer).
DO NOT make any spelling errors! The vast majority of problems come from spelling errors.
If you find yourself struggling to get a new account working, delete the account completely, then restart Outlook and try again.
From within Outlook:
http://smallbusiness.chron.com/connect-zimbra-calendar-outlook-60726.html
Tested on Outlook 2013 using EAS (Exchange Active Sync) and should be same for 2016. Outlook version must be 2013 or higher.
First you must share a calendar using the Zimbra Web Client (ZWC):
After the calendar is configured correctly and tested, copy the URL to be used in Outlook:
Download: https://mail.yourdomain.com/zimbra/downloads/index.html
User Guide: https://www.zimbra.com/docs/ne/8.6.0/zcs_connector_for_user_guide
Admin Guide: https://www.zimbra.com/docs/ne/8.6.0/zcs_connector_for_outlook_guide
The connector will run on various versions of Windows and with various versions of Outlook, but you should check before installing. See the first pages of either manual.
Newer versions of Outlook 2016 may not present the EAS account type during account setup.
See also Troubleshooting Outlook