Outlook: Opening a Mailbox (Outlook Web App + New Outlook) What will be covered: How to open a mailbox 1. Open Outlook Web App Navigate to the Office 365 portal ( https://outlook.office365.com ) and log in with your credentials. 2. Open Shared Mailbox Click on your profile picture or initials in the top-right corner, Select Open another mailbox . Lastly Enter the email address of the shared mailbox and click Open. The shared mailbox will open in a new tab.