Open Outlook (desktop) and complete the following steps:
- Click File > Account Settings > Account Settings.
- On the Email tab in the list, click the Exchange account type, click Change
- Ensure Use Cached Exchange Mode is ticked
- Click More Settings
- On the Advanced tab, under Open these additional mailboxes, click Add, and then enter the mailbox name of the person whose mailbox that you want to add to your user profile
- The new mailbox should show up in the folder list under your other accounts
Outlook Web (OWA)
- Login to your account at outlook.office.com
Right-click your name in the folder list, and click Add shared folder.
In the Add shared folder dialog box, type the name of the mailbox that you have been provided access, and click Add.
The mailbox appears in your Outlook Web App folder list.