Accessing additional mailboxes

Question

I was recently given access to another mailbox within Outlook. I went through the process of adding the mailbox to my account and restarting outlook. Once the mailbox had been added to my account, I would receive an error message saying, "The set of folders cannot be opened. The attempt to log on to Microsoft Exchange has failed." when I click on the mailbox. Why am I not able to access this other mailbox?

Answer 1

Dear @Kelli Steely, 

Thank you for outlining the issue. Based on the error you see right after adding the other mailbox, the most common causes are permission or profile configuration issues. 

Please see the following workarounds that will help you verify the issue:  

(Note: Since your post is tagged for the new Outlook on the web, the instructions above reflect that version. If you are using classic Outlook or new Outlook desktop app instead, feel free to tell me and I will provide the exact steps.) 

Option 1: Verify mailbox access through Outlook on the web if you are using the new or classic Outlook desktop application 

Try opening an InPrivate or Incognito browser window, or clear your browser cache first. 

Sign in to office.com > Apps > Outlook.  

Select your profile picture in the top right corner. 

Choose Open another mailbox. 

Enter the email address of the mailbox you were given access to. 

If the mailbox still does not open, please remove it and follow the steps below. 

Option 2: Confirm the mailbox is added correctly in classic Outlook 

If you are using the new Outlook desktop app, please switch temporarily to classic Outlook and follow these steps: 

Open Classic Outlook. 

Go to File, then Account Settings. 

Select your primary account and choose Change. 

Select More Settings, then open the Advanced tab. 

Under Mailboxes, select Add and type the mailbox name. 

Save the changes and restart Outlook. 

Please make sure the mailbox is not added as a second Exchange account. If it appears more than once, for example once from automatic mapping and once manually, remove the duplicate so it is listed only a single time. 

Option 3: Reach out your organization IT admin 

Please ask the administrator to remove your Full Access permission from the mailbox and then add it back via the Exchange Admin Center by navigating to Recipients > Mailboxes, selecting the mailbox, opening Mailbox delegation and editing Full Access to remove and re‑add your account. 

After the change is applied, please wait up to 60 minutes, then close Outlook completely and reopen it. 

As community moderators, we kindly ask for your understanding that our access to internal development details is limited. Our primary role is to guide users toward the appropriate resources and support channels. While we may not have visibility into performing deeper backend analysis, we’ll continue doing our best to support you within the scope of our responsibilities.  

I hope this information is helpful. If you have any questions or need further support, please don’t hesitate to reach out at any time. 

Thank you for your patience and understanding. 

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Answer 2

The error occurs because the permissions on the other user’s mailbox folders are incomplete. Even if access was granted to a specific folder (for example, their Inbox), access must also be granted on all parent folders up to the top of the mailbox.

To fix this, the mailbox owner (or an admin) must:

1. In Outlook, right‑click the mailbox root (for example, “Mailbox – User B”) → Folder Permissions, and grant at least Folder visible permission to the user who is trying to open the mailbox.
2. Right‑click the parent folders in the path (for example, Inbox) → Properties → Permissions, and grant appropriate permissions there as well.
3. Ensure the specific subfolder (for example, Invoices) also has the required permission level.

After the correct permissions are set on the mailbox root, all parent folders, and the target folder, the mailbox can be opened without the “The set of folders cannot be opened” / “attempt to log on to Microsoft Exchange has failed” error.

For more detailed guidance on how to delegate and access another user’s mailbox, see:

• “Allow someone else to manage your mail and calendar”
• “Manage another person's mail and calendar items”

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References:

• Unable to open default folders-you do not have permissions to logon error when accessing another user's mailbox folder