User cannot access some delegated mailboxes

Question

A user wanted to have full access to multiple mailboxes. We gave them the "Read and Manage" Permissions in the 365 Admin Center.

Now we have the issue that the user cannot open some of these mailboxes when they try to open them in outlook classic it throws an error saying something like "Folder cannot be expanded. A client process failed".

We tried opening the mailboxes in outlook.com error pops up saying access denied.

We tried using Outlook new, also doesnt let us open them even though it says we have the permission when we add the mailboxes.

The user uses online mode for shared mailboxes. We have not tried downloading these mailboxes because it would take ages to load and the outlook data file would definitly use more than 50GB afterwards.

For information: We cant use the exchange powershell because we dont have a valid subscription.

Answer 1

Hi Conrad, Johannes, 

Thank you for posting your question in the Microsoft Q&A forum. 

Based on the information you've provided and my research into the error message “Folder cannot be expanded. A client process failed”, I’d like to offer a few suggestions you might consider trying: 

1. Permission propagation delay 
   Since you've already tested access across multiple Outlook platforms (Classic, Web, and New Outlook) and still encounter errors, it's possible that the issue lies in the shared mailbox permissions not fully propagating across Microsoft 365 services. Microsoft notes that permission changes can take up to 60 minutes to take effect. If it's been longer and the issue persists, you might try removing the user from the mailbox permissions and re-adding them to refresh the delegation. 
2. Check Email Apps settings 
   In the shared mailbox settings (via Microsoft 365 Admin Center), check the Email apps section to ensure that the necessary apps (especially Outlook) are enabled for the mailbox. If Outlook is disabled here, it could prevent access. 
3. Verify mailbox visibility 
   Confirm whether the shared mailbox is hidden from the address list. If it is, this can sometimes interfere with access, especially in Outlook Classic or when trying to add the mailbox manually. 
   
4. Test with another user 
   If all configurations appear correct and the issue persists, consider delegating access to another user temporarily to test whether the issue is specific to the original user or more broadly related to the mailbox itself. 

Let me know if you'd like help walking through any of these steps or if you need further assistance. 

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Answer 2

You don't require any advanced licensing to use Exchange PowerShell. What license are you using currently?