Office 365 (Outlook 2013 / 2016 / 2019 for Win) - Use and Configure the Reading (Preview) Pane
When you select a message in the message list, you can preview the message in the Reading Pane and access many different types of file attachments. You can also customize how the Reading Pane works and appears, or you can turn it off.
Turn the Reading Pane OFF or ON
To turn the Reading Pane OFF:
On the View tab, in the Layout group, select Reading Pane in the ribbon, and then select Off. Selecting Off closes the Reading Pane only for the folder you’re in. To turn off the Reading Pane quickly for multiple folders, on the View tab, select Change View > Apply Current View to Other Mail Folders, and then choose folders in the Apply View dialog box.
To turn the Reading Pane ON or to MOVE it:
On the View tab, in the Layout group, select Reading Pane, and then select Right or Bottom.
Reading Pane Options:
On the View tab, in the Layout group, select Reading Pane in the ribbon, and then select Options.