If your messages are being marked as read as you browse the Inbox, you need to change the Reading Pane options and disable Mark as read when the selection changes.
Go to Tools, Options, Other tab, Reading pane options button. Remove the check from Mark as read when the selection changes. This dialog also contains and option to mark messages read after viewing them for a period of time change and single key reading with the space bar. This handy option allows you to page through a message to read it. When you get to the end of the message, the space bar will move on to the next one.
My preference is to disable both mark as read options and use Ctrl+Q to mark messages read as I go along. (You can also right click and choose Mark as Read). When I’m reading then deleting the message, I press Del key and don’t bother marking the message as read.