How to Fix Outlook Shared Mailboxes Not Showing or Disappearing After 30 Seconds

A practical step-by-step guide for Microsoft 365 environments where shared mailboxes fail to appear in Outlook, or briefly appear and then vanish shortly after Outlook opens.

We recently worked through an issue where Outlook would either not show shared mailboxes at all, or the shared mailboxes would appear briefly and then disappear roughly 30 seconds after Outlook launched.

In these cases, it is worth testing mailbox assignment first from Microsoft 365 Admin, but the confirmed fix was an Outlook Autodiscover registry change on the affected user profile.

Confirmed fix: Add the ExcludeLastKnownGoodUrl DWORD under the user’s Outlook Autodiscover registry path and set it to 1, then restart Outlook.

Common Symptoms

• Shared mailboxes do not appear in Outlook at all.
• Shared mailboxes appear when Outlook opens, then disappear after around 30 seconds.
• The mailbox is confirmed as assigned in Microsoft 365, but Outlook still does not keep it visible.
• OWA may work normally while the Outlook desktop app does not.

Step 1: Remove and Re-Add the Shared Mailbox in Microsoft 365 Admin

Before making registry changes, confirm the shared mailbox assignment is healthy from the Microsoft 365 side.

1. Open the Microsoft 365 Admin Center.
2. Go to Teams & Groups or Shared Mailboxes, depending on the portal layout.
3. Open the affected shared mailbox.
4. Remove the affected user’s access permissions.
5. Wait a few minutes for the change to apply.
6. Re-add the user to the shared mailbox.
7. Close and reopen Outlook and test again.

If the issue still remains after re-adding the mailbox, proceed with the registry fix below.

Step 2: Apply the Confirmed Registry Fix

This is the confirmed method that resolved the issue for affected users.

Registry Path

HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Autodiscover

Create the Following DWORD

Name: ExcludeLastKnownGoodUrl
Type: DWORD (32-bit)
Value: 1

What to Do

1. Close Outlook fully.
2. Open Registry Editor as the affected user.
3. Browse to:
   HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Autodiscover
4. Create a new DWORD (32-bit) Value called ExcludeLastKnownGoodUrl.
5. Set the value to 1.
6. Close Registry Editor.
7. Reopen Outlook and test.

Important Note About the Office Version Path

The registry path uses the Office version number in place of x.0. In most modern Microsoft 365 Apps environments, this will usually be:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Autodiscover

If the Autodiscover key does not already exist, it can be created manually.

Why This Helps

This change tells Outlook to stop relying on the Last Known Good URL for Autodiscover in the affected user profile. In environments where Outlook is incorrectly resolving or caching mailbox discovery data, this can stop shared mailboxes from disappearing after launch.

Testing After the Fix

1. Open Outlook.
2. Wait at least 1 to 2 minutes to confirm the shared mailbox remains visible.
3. Expand the shared mailbox and verify folders load normally.
4. Test opening Inbox, Sent Items, and any required subfolders.
5. Confirm sending from the shared mailbox if the user has send permissions.

Recommended Troubleshooting Order

1. Confirm the user still has the correct shared mailbox permissions in Microsoft 365.
2. Remove and re-add the shared mailbox assignment in the Microsoft 365 Admin portal.
3. Restart Outlook and test.
4. Apply the ExcludeLastKnownGoodUrl registry fix.
5. Restart Outlook and test again.

Best practice: Treat the Microsoft 365 remove/re-add step as a clean permission refresh, then use the registry adjustment when Outlook still drops the mailbox after launch.