Quote:Outlook’s Reading pane—a.k.a. Preview pane—displays the text of a message you’ve selected, preventing you from having to open the actual message to work with it. Here’s how to customize the Reading pane to suit your needs.

Outlook comes with several different panes, including those you see by default—the Navigation pane, for example—and others you might not bother with much—like the To-Do and People panes. Each of these is designed to make it easier to find, see, and manage things in Outlook. We’re going be taking a look at these panes throughout several articles, showing you how to access, work with, and customize them. And we’re starting with the Reading pane.

The Reading pane is enabled by default. When you click on a message in any folder, the pane displays the contents of that message, along with basic controls for replying and forwarding the message.

Full reading: https://www.howtogeek.com/394790/how-to-customize-the-reading-pane-in-outlook/