1. Login to https://portal.office.com using your own credential.
2. Once done sign in into the portal. choose Outlook.
3. On your top-right-hand, you can see your picture/name. Click on it and choose Open another mailbox.
4. You will see below pop-out box. Key-in the email address that you want to access. Then click Open and you can see all the emails in this mailbox.