Shared Mailbox

Disable Download Shared Folders in classic Outlook on one affected computer; this makes the shared mailbox read directly from Exchange instead of the local Outlook cache and is the most relevant test when new Outlook works but classic Outlook intermittently loses the mailbox.

Test without shared-mailbox caching

1. In classic Outlook, select File → Account Settings → Account Settings.
2. Select the Exchange account associated with the shared mailbox, then select Change.
3. Select More Settings → Advanced.
4. Under Cached Exchange Mode Settings, clear Download Shared Folders.
5. Select Apply → OK → Next → Done, then restart Outlook.
6. Open the shared mailbox and monitor whether it remains visible. When a shared mail folder is selected, the Outlook status bar should show Online.

This setting avoids synchronization problems involving the local OST cache. The trade-off is that shared-mailbox email will not be available offline, and performance will depend more directly on network connectivity.^1^

If this fixes the problem across repeated tests, an administrator can deploy Disable shared mail folder caching through Group Policy rather than changing each computer manually.^1^

If the entire mailbox still disappears

Create a clean classic Outlook profile:

1. Close Outlook.
2. Open Control Panel → Mail → Show Profiles → Add.
3. Create and configure the new profile.
4. Hold Shift while starting Outlook.
5. Select the new profile in Choose Profile, and test the shared mailbox.

If the mailbox remains stable in the new profile, the original profile is damaged and should no longer be used. Microsoft identifies creating a new profile as the way to test for classic Outlook profile corruption.^2^

Microsoft also documents an older shared-mailbox synchronization condition in which Outlook remains on Updating this folder. For that specific symptom, a new profile resolves an already-corrupted profile even though the underlying service issue was fixed.^3^

Check for the Version 2505 known issue

If the folders specifically flicker, move, or disappear while moving messages, check File → Office Account → About Outlook.

That behavior affected classic Outlook Version 2505, starting with Build 18827.20128. Microsoft fixed it through a service change; restart Outlook to receive the change. Disabling Download Shared Folders is also the documented workaround for that condition.^4^

Additional conditional checks

• Many shared mailboxes or folders: Reduce the number opened in classic Outlook. Synchronization reliability depends on mailbox size, folder count, hardware, and network performance.^5^
• No improvement with a new profile: Run the classic Outlook troubleshooter through Windows Get Help. These troubleshooters apply to classic Outlook, not new Outlook.^6^
• Multiple users are affected: Have the Microsoft 365 administrator compare the affected classic Outlook versions and open a Microsoft 365 support request with timestamps, screenshots, Outlook build numbers, and whether disabling shared-folder caching corrected the behavior.

Avoid registry editing unless directed by support; the safer first remedies are disabling shared-folder caching and testing a new Outlook profile.

References

1. Manage download settings for shared mail folders in Cached Exchange mode in Outlook
2. [Classic Outlook](https://support.microsoft.com/outlook/getstarted/outlook-for-windows-not-responding-hangs-freezes-or-stops-working)
3. Shared mailbox is not automatically refreshing for new email in Outlook | Microsoft Support
4. Shared mailbox folders flicker and move around when trying to move items to the folders | Microsoft Support
5. Outlook performance issues when you have many shared folders or mailboxes open
6. Resolve Outlook for Windows issues with automated troubleshooting tools | Microsoft Support