By default, any user can't see another users' mailbox. In specific scenarios, where requested to IT, we can enable this option between specific individuals.
Once enabled by IT, follow these steps:
1. In Outlook Web, click on your profile picture/Named Initials on the top-right of the screen and select Open another Mailbox from the drop-down menu.
2. On the center of web browser window, start typing the name of the other user's mailbox, click on their Full Name and then Open:
A new browser tab will open. Please keep in mind that if you reply from this tab, it will do it as the user mailbox that is open, not from your account!