To access a shared mailbox to view the messages (or perhaps setup the out of office message on that mailbox). Since the mailbox doesn't have a known password; you just sign in with your own account that can access it, and then switch to that account. You can switch back and forth between any accounts the same way as long as you have access to them. Here are the steps:

  1. First sign into the exchange link at the top of the website with your regular username and password.
  2. click on your name which will appear near the top right corner. If you look at the attached picture, you will see ours says "Student Helpdesk" and I had clicked on that.
  3. type in the email address for the shared mailbox (you have access to) within that popup and click "open"
  4. that will open up the shared mailbox in exchange. 
  5. Now you can view those messages for that email address, or perform other functions such as click "options" and "set automatic replies" which is also at the top right corner just below where the name appears.
  6. fill any of those out if desired and save it.
  7. You can switch back to your own mailbox by performing the same steps 2 through 4 as above.