It is not possible by default, but you can download a plugin for the Mail app. The plugin is called TruePreview. It is said that it supports OS X Mountain Lion and below, but I had it working on my Mavericks machine.
Anyway, here's the method:
- Quit the Mail app
- Download TruePreview (scroll down for latest versions) and run the
- Launch Mail.app and open Preferences, click on the >> arrow and
- Adjust the Mark as Read settings as desired
Have a read on this page and you'll find that there is a Terminal command that can be used to delay the Mark As Read setting. However, I'm not sure you can disable it completely.
defaults write com.apple.Mail MarkAsReadDelay 2
Here, 2 is a measurement of the number of seconds to delay.
defaults write com.apple.Mail MarkAsReadDelay
The above will just revert the setting back to its default state.
Alternatively, if you're okay with marking it as unread, an easier option would be to use a keyboard shortcut instead of manually marking as unread. The standard keyboard shortcut, which toggles “read” and “unread”, is cmd+shift+u. You can also go to System Preferences → Keyboard to customise your own keyboard shortcut.
Have a look, see which options takes you where, and stick with whatever is comfortable. Let me know if anything went wrong during the plugin installation (that is, if you did install it).