Outlook on the web shows delays in folder synchronization for shared mailboxes

Hi @Jenson

Thank you for posting your question in Microsoft Q&A. 

As your description, we understand that you are encountering sync issue in OWA. Since OWA is a browser-based client that relies on real-time server communication rather than local storage files. However, OWA can experience delays due to browser-side caching of session data, network latency, or server polling intervals (updates may take 5-15 minutes to propagate across users in some cases, especially for shared mailboxes). If desktop Outlook syncs fine, the issue is likely isolated to browser behavior or OWA's refresh mechanics. 

To troubleshoot sync speed issues in OWA, could you try to follow these steps: 

Test in incognito/private mode: Open OWA in a private browsing window. This bypasses existing browser cache, cookies, and extensions. Log in, access the shared mailbox (via "Open another mailbox" or by adding shared folders), and check if loading/sync is faster or if changes appear promptly.  

If it improves , the issue might be related to browser session. You could try  to: 

Clear browser cache for outlook.office.com ( Settings > Privacy > Clear browsing data, select “Cookies” and “Cached images/files,” filter for “office.com”). 

Try a different browser to see if the issue still persists. 

To better assist you, could you provide additional information for the following questions: 

1.Are multiple shared mailboxes connected to your account?

2.Which version of Outlook desktop are you currently using (Classic Outlook or the New Outlook)?  If you're using Classic Outlook, it may be related to how local caching works. Classic Outlook uses Cached Exchange Mode, which stores a local copy of your mailbox data (including shared mailboxes) in an Offline Storage Table (.OST) file. This allows Outlook to quickly display messages and folder changes from the local cache, while syncing updates with the Exchange server in the background. Even if server sync is slow, users often don’t notice delays because they’re interacting with the cached local copy. 

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Hello Jenson,

Unlike the Outlook desktop client, Outlook on the web (OWA) does not have a local client-side cache for shared mailboxes that you can clear manually. All data is accessed and processed directly on the server. Therefore, shared mailbox sync issues in OWA point to a problem on the server side or with your browser session, not a local cache. 

Steps to troubleshoot OWA sync issues

Because the problem is isolated to OWA, focus on these steps to reset the connection and clear any session-related data. 

For the user experiencing the sync delay:

• Sign out and close the browser. Log out of your Microsoft 365 account, close all browser tabs, and then fully exit the browser application. Open a new session and log back in.
• Clear your browser's cache and cookies. Corrupted browser data can interfere with how OWA loads shared mailboxes.
  • Open your browser's settings.
    • Find the option to clear browsing data, cache, and cookies.
      • Select an appropriate time range, such as "All time."
        • Restart your browser after clearing the data.
        • Test in a private browsing window. Open a private or incognito window and log into OWA. This will create a fresh session without any extensions or cached data. If the shared mailbox works correctly, an extension or cached setting in your normal browser profile may be the cause.
        • Test with a different browser. If the issue persists in a private window, try using a completely different web browser (e.g., Firefox or Chrome) to access OWA. This can help rule out any browser-specific problems. 

For the administrator: If the issue affects multiple users, an administrator should investigate potential server-side problems.

• Check Microsoft 365 service health. Check the Service Health dashboard in the Microsoft 365 admin center for any active incidents or advisories affecting Exchange Online.
• Verify mailbox permissions. Confirm that the affected users have the correct permissions for the shared mailbox. A good method is to temporarily remove and then re-add their "Full Access" permission.
  • Go to the Exchange admin center.
  • Navigate to Recipients > Mailboxes and select the shared mailbox.
  • In the Mailbox delegation section, find the affected user and remove their "Full Access" permission.
  • Wait a few minutes for the change to replicate.
  • Add the user's "Full Access" permission back.
  • Advise the user to test the mailbox in OWA again after some time for the changes to propagate.
• Remove and re-add the shared mailbox from the user's view. The admin can remove the automapped shared mailbox for the user, forcing them to open it manually. This can reset the server-side connection for that user's specific OWA session.
  • Instruct the user to right-click the shared mailbox in their OWA folder list and select Remove shared mailbox.
  • Then, have the user manually re-add it by clicking their name in the top-right corner, selecting Open another mailbox, and typing the name of the shared mailbox.

Regards,

Raviteja M.