Execute the following commands in a Terminal window, paying great attention to the exact spelling, then press return.
defaults write com.apple.mail EnableBundles 1
defaults write com.apple.mail BundleCompatibilityVersion
Random ramblings
Execute the following commands in a Terminal window, paying great attention to the exact spelling, then press return.
defaults write com.apple.mail EnableBundles 1
defaults write com.apple.mail BundleCompatibilityVersion