Outlook, auto mark as read after selection

When you select a message in outllok it becomes automatically "marked as read".

You can change some features related to it in:

Tools menu > Options > Other tab > Reading Pane button

You have some available options very self explainable.
Check or uncheck as best fits your requirements.

“Mark items as read when viewed in the Reading Pane”.
You might also want to change the number of seconds in this dialog.
“Mark item as read when selection changes”