O que você está procurando é um plug-in chamado TruePreview . Na home page:
One of the most common shortcomings/omissions/bugs/failures in Mail is the inability to truly preview messages. If the preview pane is displayed in the message viewer window, as soon as a message is selected and displayed, it is marked as read. Most every other e-mail client on the planet provides an option to delay marking messages as read. This particular issue has been the one nagging annoyance that has bothered me for years now. With every OS X upgrade, I have anticipated Apple finally adding this necessary functionality, only to be disappointed.
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