In order for you to open a shared mailbox, you must have permission to the shared mailbox. If you need to get permission to the mailbox, please contact DES IT Support.
Login to Office 365 Outlook using your DES account email address and password.
Click on your display photo (or initials if you don't have a photo uploaded) in the upper-right corner of the window and click “Open another mailbox” as shown in the screenshot below:
The “Open another mailbox” prompt will now appear. Type the email address of the mailbox you want to view in here and click “Open” as shown below:
The new mailbox will open a new tab/window.
Everything you do from this mailbox will be acting as the mailbox and not your personal DES account. Using this method gives you more options than using the Outlook client on your PC to administer the mailbox account.