Hi, I need some help to troubleshoot a calendar synchronization issue in a Zimbra + Outlook + CalDAV environment.

[jose A]

We have a Zimbra Open Source Edition installation, version 8.8.8_GA_3025.FOSS (build 20180906060045). One user works primarily with Outlook 2016 (also tested with Outlook 2019 with the same results) configured as IMAP, and also uses an iPhone with the native Calendar app connected via CalDAV to sync appointments and meetings.
The iPhone is running the latest version of iOS.


Main Problem:


Meetings created or modified in Outlook behave inconsistently when syncing with the Zimbra server and the iPhone, resulting in:

• Duplicate invitations being sent to other participants.
• In some cases, Outlook does not replicate calendar updates to the iPhone (via CalDAV).

When the user modifies or accepts an invitation in Outlook, sometimes:

• The update never reaches the iPhone,
• Or a new duplicate invitation is sent,
• Or the event partially updates, but the original invitation is not removed.

In contrast, if the user creates or modifies an event on the iPhone, synchronization works correctly in both directions (iPhone → Zimbra → Outlook).


Additional Behaviors Observed:

• Invitations sent from Outlook sometimes need to be manually accepted on both platforms (Outlook and iPhone), as if they were different events.
• Some modifications made in Outlook do sync correctly, but others create duplicates without removing the previous entry.
• From the iPhone side, all changes behave as expected.

Additional Context:

• Email is configured via IMAP because Outlook does not support EWS/ActiveSync with Zimbra OSE.
• The iPhone calendar is connected directly to Zimbra using CalDAV.
• ZCO (Zimbra Connector for Outlook) is not being used because it is not compatible with the Open Source Edition.

Question / Request for Help:


Has anyone experienced this kind of duplication and desynchronization issue between Outlook + IMAP + CalDAV on Zimbra OSE?
Are there any known recommendations, patches for version 8.8.8, alternative configurations, or best practices to prevent duplicates and achieve more reliable synchronization?
Would upgrading to a newer version of Zimbra OSE or applying specific CalDAV/IMAP adjustments be advisable?


Any guidance would be greatly appreciated.

 

[MarineBand5524]

Check the plugin's profile -- settings -- advanced -- scheduling

let the server handle scheduling an disable Outlook from sending invitations automatically (uncheck the option to send meeting requests from Outlook or avoid mapping attendees if needed)

see if that will help you.

Is there a reason you can't update Zimbra? Have you tried the Outlook CalDAV Synchronizer?