How to fix Outlook 'Cannot expand folder' errors when opening shared mailboxes

Summary

This guide helps you troubleshoot and resolve: How to fix Outlook 'Cannot expand folder' errors when opening shared mailboxes. Follow the steps below to fix the issue.

Common Causes

The 'Cannot expand folder' error in Microsoft Outlook typically occurs when the application loses its synchronisation link with a shared mailbox or when the local data file (.ost) has become corrupted. This prevents you from viewing the subfolders of a shared mailbox, even if you have the correct permissions.

Quick Fix Steps

Before attempting more complex repairs, try these rapid solutions to refresh the connection to the Microsoft 365 server:

• Restart Outlook: Completely close Outlook and restart the application to force a fresh connection.
• Check Outlook Web Access (OWA): Log in to outlook.office.com. If you can expand the folders there, the issue is with your local Outlook installation, not your account permissions.
• Update Outlook: Go to File > Office Account > Update Options > Update Now to ensure you are on the latest build.

Detailed Instructions

Method 1: Disable Download Shared Folders

Outlook often struggles to cache large shared mailboxes. Disabling the local caching of these folders forces Outlook to pull data directly from the server, which often resolves expansion errors.

1. Open Outlook and go to the File tab.
2. Select Account Settings and then click Account Settings again from the dropdown.
3. Select your primary email account and click Change.
4. Click More Settings in the bottom right corner.
5. Navigate to the Advanced tab.
6. Under the 'Outlook Store' section, uncheck the box labelled Download shared folders.
7. Click OK, then Next, and finally Finish.
8. Restart Outlook for the changes to take effect.

Method 2: Update the Offline Address Book (OAB)

If the shared mailbox is linked to an outdated entry in your address book, Outlook may fail to locate the folder path.

1. Go to the Send / Receive tab on the top ribbon.
2. Click Send/Receive Groups.
3. Select Download Address Book.
4. Ensure Download changes since last Send/Receive is checked and click OK.

Method 3: Recreate the Outlook Profile

If the local .ost file is corrupted, the most reliable fix is to create a new profile. This will force Outlook to rebuild the local cache of all mailboxes.

1. Close Outlook completely.
2. Open the Control Panel and search for Mail (Microsoft Outlook).
3. Click Show Profiles.
4. Click Add and give the new profile a name (e.g., "Outlook New").
5. Follow the prompts to enter your email address and password to configure the account.
6. In the 'When starting Microsoft Outlook' section, select Always use this profile and choose your new profile from the dropdown.
7. Click OK and restart Outlook.

Common Related Issues

• If you are experiencing general synchronisation delays, refer to our guide on Troubleshooting Outlook Send/Receive Errors.
• If you need to move your local data to a new profile, see Backing Up and Restoring Outlook Data Files (.pst/.ost).
• For new users setting up their environment, please follow Setting Up an Email Account in Outlook (POP/IMAP/Exchange).

If these steps do not resolve the issue, please Open a Ticket so our technical team can analyse your account permissions and server logs.

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