How to Fix Outlook Shared Mailboxes Not Showing or Disappearing

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

We recently worked through an issue where Outlook (classic) would either not show shared mailboxes at all, or the shared mailboxes would appear briefly and then disappear again within 24hr window. Even after a Outlook profile rebuild.

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 (classic).

This change tells Outlook (classic) 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.

Common Symptoms

  • Shared mailboxes do not appear in Outlook (classic) at all.
  • Shared mailboxes appear when Outlook (classic) opens, then disappear again.
  • The mailbox delegation permissions are confirmed in Microsoft 365 Admin Centre, but Outlook still does not keep it visible.
  • OWA may work normally while the Outlook (classic) desktop app does not.

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

How to Steps

  1. Close Outlook (classic) 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 (classic) and test.

Field Note

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.

 

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