Apple Mail under OS X Leopard doesn’t support the GrowlMail Mailbundle which comes as an extra when installing the Growl notificaiton framework. Since I really missed that feature, that has been with me as long as I own a Mac, I searched around the WWW to find a solution.
Luckily there is a way:

• Quit Apple Mail
• Download the Growl .dmg from its website and install GrowlMail
• Launch the Terminal
• Enter the following two commands
  defaults write com.apple.mail EnableBundles -bool YES
  defaults write com.apple.mail BundleCompatibilityVersion -int 3
• Start Apple Mail and enable the GrowlMail plugin
• Be happy ever after 🙂

via: blimps are cool