1.  Click on "File".

  2. Then click on "Add account".

  3. Type in the email address and continue with "Next".

  4. If you are using a desktop device, select "Change account settings", otherwise continue with "Finish".

  5. If you selected the "Change account settings" option, unselect "Use Exchange cache mode" and continue with "Finish".

  6. Complete this step by restarting Outlook.

After that, the mailbox will appear on the left side under your mailbox with the email address as the name.