By default, Outlook can mark an email as read simply because you selected it in the Reading Pane.
If you prefer messages to remain unread until you manually mark them as read, you can change this setting.
Open Outlook and go to:
File > Options > Advanced
Under Outlook panes, click:
Reading Pane
Untick:
Mark items as read when viewed in the Reading Pane
Also untick:
Mark item as read when the selection changes
Click OK, then OK again.
Outlook will no longer automatically mark emails as read simply because you select or preview them.
You can still mark a message as read manually by right-clicking it and selecting Mark as Read.