When using Outlook, it offers a 'Reading Pane' function that allows you to read the selected email in the same window (not required to double click on it to open it). After this, the email is marked as read and you won't need to read it again, which is perfect.
However, if you select accidentally an email this will be marked as read when you didn't want to read it, to avoid this you can avoid the 'Reading Pane' or you can change how this 'Mark as read' function works. To do the last one, go to 'File' and select 'Options'.
In 'Options', go to 'Mail' and then locate 'Outlook panes' section, click on 'Reading Pane...' and there you can disable the behavior explained before, it will require to open the mail with double click to mark it as read.
Once you have this modified, you won't have more mails read by accident.