You already know that after keeping one email open for a while or moving up and down in your inbox can make certain emails appear as read, while others are still unread. This also changes the color they’re marked with, which can get confusing when you mistakenly marked an email as read, although you didn’t have time to go through it just yet.
To avoid such situations, you can go to File, then choose the Options tab and click on Mail. To your right, you will see a Reading Pane checkbox that you should click on to view the various options. You can choose to check and uncheck any of them, as well as change the number of seconds before marking an email as read. Once you’re done customizing, click OK to save your options.