By default, Outlook is configured to mark a message read a few seconds after the message is selected. This can result in messages accidently marked as read.
To disable this option, go to Tools, Options, Other tab, Reading pane options.
Reading Pane Options Tutorial
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My preference is to never automatically mark messages read. They will be marked as read if I double click to open or Reply/Forward. I use Ctrl+Q to mark other items read as I read them. (Use Ctrl+U to mark unread)
You can also open the Reading pane options dialog if you very carefully right click on the edge of the reading pane. (This was easier to do with the preview pane in older versions of Outlook.)
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Last reviewed on Aug 31, 2011
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