With OS X 10.8.4, Apple has updated Mail, which means that plug-ins that are not up-to-date won't work, and will be placed in a ~/Library/Mail/Bundles (Disabled) folder. In order to get any plug-ins to work, you'll need to find the new UUIDs and add them to the plug-ins' info.plist file.
We ran a hint about this back in 2010, but it's good to remind people how to do this. You want to run two commands in Terminal to get two different UUIDs:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID
<key>SupportedPluginCompatibilityUUIDs</key> <array> <string>2183B2CD-BEDF-4AA6-AC18-A1BBED2E3354</string> <string>19B53E95-0964-4AAB-88F9-6D2F8B7B6037</string> <string>2DE49D65-B49E-4303-A280-8448872EFE87</string> <string>1146A009-E373-4DB6-AB4D-47E59A7E50FD</string> <string>6E7970A3-E5F1-4C41-A904-B18D3D8FAA7D</string> <string>EF59EC5E-EFCD-4EA7-B617-6C5708397D24</string> <string>4C286C70-7F18-4839-B903-6F2D58FA4C71</string> <string>608CE00F-4576-4CAD-B362-F3CCB7DE8D67</string> </array>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130606035546805