Exchange Online shared mailbox not updating new emails in Outlook desktop

Question

Hello

We’re experiencing an issue with a shared mailbox in Exchange Online where new emails are not syncing properly in the Outlook desktop app (Microsoft 365). The shared mailbox appears in the user’s profile, but newly received messages only show up after manually restarting Outlook or switching folders. Interestingly, the same mailbox syncs correctly in Outlook on the web (OWA), so the issue seems isolated to the desktop client.

The shared mailboxes are added by users being placed into the mailbox, so removing it to add it again is not really an option for 100's of mailboxes.

Answer 1

Hello @Sam Molyneux,

Welcome to Microsoft Q&A Forum!

Thank you for your detailed information. If you prefer not to remove and re‑add the shared mailbox, here are a few steps I was able to find that may help improve the syncing behavior. Please try them when you have a moment and let me know how it goes. 

1/ Disable offline caching (so Outlook always syncs the newest emails) 

• Open Settings (gear icon) in New Outlook
• Go to General > Offline 
• Toggle Offline settings to Off > Click Save 

This forces Outlook to sync live data directly instead of using an outdated cache.

2/ Manually trigger a sync 

Go to View > Click Sync.  

This immediately forces Outlook to refresh all folders, including shared mailboxes. 

3/ Make sure TLS 1.2 and 1.3 are enabled 

Press Win + R, type inetcpl.cpl and press OK to open Internet Properties.  

• Go to the Advanced tab, scroll down to the Security section.  
• Ensure that Use TLS 1.2 and Use TLS 1.3 are enabled.  
• Click OK and restart your Outlook app.  

Please understand that my initial response may not always resolve the issue right away. However, with your help and more detailed information, we can work together toward the right solution.  Please feel free to update me with the outcome of these steps, and I will be happy to continue assisting. 

Kind regards, 

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Answer 2

The issue you're experiencing with the shared mailbox in Exchange Online not syncing new emails in the Outlook desktop app can be related to several factors. Here are some potential solutions:

1. Clear Offline Items: For cached Outlook clients, you can try clearing offline items from the folder where the sync failures occur. Right-click the folder in question, select Properties, then the General tab, and choose Clear Offline Items.
2. Switch to Online Mode: If the problem persists, consider using Outlook in online mode instead of cached mode. This can help ensure that changes are reflected immediately.
3. Reduce the Number of Folders: If the shared mailbox has a large number of folders, consider reducing them. This can improve synchronization performance, especially in environments with Office 365.
4. Check for Updates: Ensure that the Outlook desktop app is updated to the latest version, as updates often include fixes for synchronization issues.
5. Recreate Outlook Profile: If the above methods do not resolve the issue, you may need to recreate the Outlook profile for the affected users. This can help refresh the connection to the shared mailbox.

These steps should help address the synchronization issue with the shared mailbox in Outlook desktop. If the problem continues, further investigation into the specific configuration of the mailbox and user permissions may be necessary.

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References:

• Performance and synchronization problems when you work with folders in a secondary mailbox in Outlook
• Synchronization is unsuccessful after mailbox items are changed through EWS calls in cached-mode Outlook client