How do I stop marking mail as read caused by looking at the preview pane?

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:

defaults write com.apple.Mail MarkAsReadDelay 30

(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.

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Do check for a recent version of TruePreview as older versions don't work well with Mountain lion.

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