- Open Outlook 365 and go to the File tab in the top-left corner.
- Click on the "Account Settings" button and select "Account Settings" from the dropdown menu.
- In the Account Settings window, select the email account associated with your own mailbox and click on the "Change" button.
- In the Change Account window, click on the "More Settings" button.
- In the next window, switch to the "Advanced" tab.
- Click on the "Add" button under the "Open these additional mailboxes" section.
- A dialog box will appear where you can enter the name or email address of the user whose mailbox you want to add. Type in the name or email address and click on the "OK" button.
- The user's mailbox will be added to the list of additional mailboxes. Click on the "OK" button to close the dialog box.
- Back in the Change Account window, click on the "Next" and then the "Finish" button to save the changes and exit.
- The added user's mailbox will now appear in the Outlook folder pane, and you can access it by expanding the list of mailboxes under your own mailbox.
Please note that you'll need appropriate permissions to access another user's mailbox. Additionally, the steps may vary slightly depending on the version of Outlook 365 you are using.