Accessing a Shared Mailbox in a Web Browser (Office 365 Online Portal) The following instructions will assist you in mapping a shared mailbox in https://portal.office.com. Anytime you log out of portal, you will have to complete these instructions again. The shared mailbox is only stays mapped during a single login session. Accessing Shared Mailboxes in the Office 365 Online Portal Login to https://portal.office.com and select Outlook from the list of applications. Click on your name in the top right-hand corner to open up the My Accounts menu. From the My Accounts menu, click on the Open another mailbox... option. The Open another mailbox window will appear. Enter in the email address associated with the shared mailbox and click on Open. A new tab will open in your browser. You will now see the shared mailbox and all of the folders associated with it. Sending from the Shared Mailbox To send from the shared mailbox, click on the New button. You do not need to change the address.