After updating to 10.4.1, MailEnhancer (one of the most useful additions to the OS X mail client) stopped working. But it doesn't take much to get it going again:

• rename ~/Library/Mail/Bundles (Disabled) to ~/Library/Mail/Bundles
• paste this at a Terminal prompt: defaults write com.apple.mail EnableBundles 1
• this one, too: defaults write com.apple.mail BundleCompatibilityVersion 2
• Quit and restart Mail