To disable or remove the reading pane from within your Outlook Web App (OWA):
Launch an internet browser (Internet Explorer (IE), Firefox, Chrome, Safari)
Sign in using your login credentials
Click the Settings gear wheel in the upper right side of the window
Click Display settings
Click the reading pane tab
Select the Hide reading pane option
NOTE: If you wish to disable the reading pane for all folders inside your Outlook Web App, place a check in the Apply to all folder box.
The reading pane will be removed from your mailbox leaving only the message summary. You must double-click a message to read the full message detail.