Prior to doing this, mailbox permissions need to be set by your Office 365 tenant administrator account
1. Login to https://Outlook.office.com and enter your credentials in the browser
2. Once logged in, click the avatar on the top right hand corner of the screen.
3. Once clicked, choose the Another user icon
This should provide you with a popup. Now type the name of the mailbox you wish to review. It will come up in the browser as another window.
Note: If the browser is closed you will need to do the last step again. Every time the entire browser is closed you will have to do follow the entire process again.