Hi rkut,
According to my research, I'm afraid that there is no option in Outlook or registry key to completely disable marking messages as read.
If you double click to view the email, it will always be marked as read. We can right click the message and manually mark it as unread. As a workaround, you can also check messages in Reading Panel only.
If you have more concerns on this feature, it is encouraged to visit the Outlook user voice forum:
https://outlook.uservoice.com/ to vote for a similar idea or post your own.
By the way, could we know why you want keep email unread?
Hope the information is helpful to you.
Regards,
Perry
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