If you have been provided with access to another email account, it is possible to view it from the Office 365 web portal.
1. Log into the Office 365 Web Portal at http://mail.office365.com
2. Click on your profile icon, and a drop-down menu will appear.
3. Select 'Open another mailbox…'
4. Type in the email address of the mailbox you wish to open.
5. Click 'Open', and it will open in another browser tab.