Click the gear symbol at the top-right of the screen.
Select Options and Outlook will default to the Account options.
Click Settings on the left-side navigation.
Find the Reading Pane section on the right side of the screen.
Find the statement labeled Choose When Items Should Be Marked as Read.
Click the first item, Mark the Item Displayed in the Reading Pane as Read, if you would like to have Outlook change the read/unread status while you are reading any given message. Note: This will trigger read responses that have been requested so people who track such things do not think you have deleted the mail without reading.
Change the number in the box to 1 (second).
Click the Save button at the bottom of the screen.