Hello M.B.,
The official supported setup for Zimbra is currently the IMAP/SMTP. After setting it up this way, please manually set up CalDAV and CardDAV in the Account settings to get the Contacts and Calendar to synchronize. We are sorry for the inconvenience, hopefully in the future we can support it as Exchange Web Services when Zimbra also improves its EWS setting. Thank you for your understanding.
Regards,
Maurice