Office 365 (Outlook 2016) - How to Change the Reading View in Outlook on the Web
When accessing Outlook on the Web, a user may want to change their Reading View to show the preview window in a different area of the screen, or completely turn off the preview function.
Select the gear icon in the upper right hand corner of the screen.
1. Choose Display settings under Settings
2. Select the reading pane appearance in the window based on the preference.
3. Select OK after choosing the reading pane appearance option