Office 365 (Outlook 2013 / 2016 for Win) - Make Changes to the Reading (Preview) Pane
You can preview messages in your Inbox (or other mail folders, like Sent Items or Drafts) without opening them by using the Reading Pane. You can also turn this option off if you choose.
Security: You can view messages safely in the Reading Pane. Potentially malicious scripts or attachments are not activated or opened automatically in the Reading Pane. Although most malicious attachment types are blocked by Outlook, messages and attachments from unknown or unsolicited senders should always be treated with caution.
Turn the Reading Pane off or on:
Note: You must turn the Reading Pane on or off for each of your mail folders individually. Turning it on in your Inbox, for example, will not display the Reading Pane in your Sent Items folder until you make this change for that folder, too.
Do one of the following:
• Turn off the Reading Pane
o On the View menu, point to Reading Pane, and then click Off.
• Turn on the Reading Pane
o On the View menu, point to Reading Pane, and then click Right or Bottom.