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.

enter image description here

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