I use Mountain Lion.
How do I prevent Mail.app (6.0) from marking a message as read automatically, just because I have looked at it in the preview pane?
You can change the mark as read delay to a great value, for example 30 seconds, by typing the following in the Terminal:
(NB: A value of -1 will disable marking as read from Preview if the preview window is hidden. Tested on Mail 5.2/OSX 10.7.4)
An other way to do do (without using the Terminal), is installing TruePreview. It's an 3th party app which allows you to do exactly what you expect.
Do check for a recent version of TruePreview as older versions don't work well with Mountain lion.
Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.
Would you like to answer one of these unanswered questions instead?