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.
To turn the Reading Pane OFF:
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.
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