Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi Sebeka,
To my knowledge, if the reading pane is turned on, Outlook will automatically open the message when you select the email item in the item list.
I understand it would be more intelligent if Outlook asks users whether to open the email when the download headers only option is selected. However currently Outlook doesn't have such settings to achieve this. In this scenario, as a workaround, you can go to Views > Reading Pane > Off to turn off the reading pane. Then you won't open the email even you select the email item.
Of course, if you prefer to have a notification/reminder when you select an email item after you enable download headers only option, it's recommended to share your experience and expectations with us in Outlook UserVoice website. Any ideas are valuable to help us improve Outlook and user experience, We sincerely appreciate for your kind understanding.
Regards,
Marvin