Problem to access shared mailbox's subfolder

Question

I created subfolders to which I assigned read permissions to a user. I noticed that the sharemailbox appeared in the user's Outlook, but when I try to expand it to see the two subfolders, I get an error. "Unable to expand the folder."

How do I fix this?

Answer 1

For classic Outlook, you need to make sure that every folder among the path to the subfolder in question has been shared with FolderVisible permissions. For example, if you shared a folder named "blabla" under Inbox, the full path will look something like ******@domain.com/Inbox/blabla. Both the Inbox folder and the "root" one (aka the one with your address stamped) need to be accessible in order for the subfolder to be accessible. In other words, open the properties for both Inbox and the root folder, and make sure the FolderVisible permission is ticked for the user in question, or on the Default entry. If you need more details/step by step instructions, here's a sample article that covers them: https://robert365.com/article/access-or-share-non-default-outlook-folders

Answer 2

Use the following checks and fixes, starting from the most common causes:

1. Confirm how the shared mailbox is added

• In hybrid or mixed-version Exchange environments, adding a mailbox only as an “additional mailbox” can cause folder expansion issues.
• In Outlook (classic):
  1. Go to File > Info > Account Settings > Account Settings.
  2. On the E‑mail tab, select the user’s account and choose Change > More Settings > Advanced.
  3. If the shared mailbox is only listed under Open these additional mailboxes, remove it.
  4. Instead, add the shared mailbox as a full account: File > Info > Account Settings > New, then enter the shared mailbox name, email address, and credentials (if applicable). Make sure the user has Full Access to the shared mailbox.
  This pattern is the supported fix in similar scenarios where users have Full Access but cannot expand folders or see full content.

2. Verify permissions on the shared mailbox and subfolders

• Ensure the user has at least Reviewer (read) permission on:
  • The shared mailbox root
  • The intermediate folders (if any) leading to the subfolders
  • The specific subfolders themselves
• If using Exchange/Exchange Online, confirm Full Access to the mailbox if the mailbox is added as an additional account.

3. Check for version / coexistence issues

• In environments where Exchange 2013/2016/2010 coexist or in hybrid (on‑prem + cloud), cross‑version shared access via Outlook or OWA can fail with “Cannot expand the folder” or similar errors.
• If this is a hybrid or coexistence scenario and the mailbox is cross‑premises, prefer:
  • Adding the mailbox as a full account in Outlook (as above), or
  • Using Outlook on the web’s Open another mailbox option instead of “Add shared folder” when applicable.

4. Update Exchange if applicable

• If the shared mailbox is on Exchange Server 2013 and there are known issues with shared or cross-domain access, make sure the server is updated to a current Cumulative Update (for example, CU7 or later for known shared mailbox issues).

If, after these steps, the user still sees “Unable to expand the folder,” collect:

• Whether the environment is hybrid or on‑prem only
• Exchange versions for both the user and shared mailbox
• Exact method used to add the shared mailbox in Outlook and escalate to Exchange/Outlook support with that detail.

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References:

• Users cannot expand folders or see complete calendar data even though they have full permissions
• Shared mailbox cannot be opened in Outlook in an Exchange Server 2013 environment that has multiple domains
• Can't open a shared folder in Outlook on the web in Exchange Server
• Share and access another person's mailbox or folder in Outlook