Quote:The Outlook client comes with several different panes that you can show and hide. Each of these panes is designed to make it easier to find, see, and manage things in Outlook. We’re going to take a look at how you access them, and how you can customize them to your way of working, starting with the Navigation pane.

Customize the Favorites Section

The Navigation pane—also known as the Folder pane—is the one on the left-hand side that shows folders like Inbox and Sent Items, as well as any shared mailboxes or groups.

Full reading: https://www.howtogeek.com/393870/how-to-customise-the-navigation-pane-in-outlook/