To be pedantic, the Zimbra calendar uses caldav; so devices that support this (whether natively, e.g. apple iOS/OSX, or through plugins, e.g. android, windows) can use and sync with the calendar. But yes, without any 3rd party plugins, outlook will not be able to sync with a caldav server.
Originally Posted by kruon
As example, my company has 90% usage of thunderbird+lightning plugin, which is fine for caldav (although there is a limitation of lightning being unable to have attachments with events in the calendar), so for the other 10% who insist on using outlook, we are evaluating appropriate caldav plugins for them, e.g. iCal4OL: Synchronize Outlook with Google Calendar, iCloud, EGroupware, Darwin, DAViCal and other CalDAV/CardDAV Solutions - ICAL Feeds Import/Export.
(no affiliation with iCal4OL, just giving one example out of many)
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