Delay a previewed email from being marked as read

I'm using Sierra 10.12.6 and MacMail.10.3.

How can I delay a selected email from being marked as read? Closing the preview pane works, as technically the mail is being read, but I want to keep the preview pane open as it helps when searching the content of emails. Ideally an add-on or something so that I can set the amount of time an email can be selected for before it is marked as read. Any suggestions anyone?

Thank you