Note: A newer post has been added that addresses 10.6.5.

Upgraded to OS X 10.6.4 and can’t use your plugins? It’s because Apple upgraded Mail.app to 4.3.

I use WideMail and DockStar and can’t live without them. Rather than waiting for the developers to release fixes there is a quick way to get them working. Make sure you have the latest versions of DockStar and WideMail installed. It’s best to quit Mail and then run the installers which will put the plugins in the proper places rather than copying the disabled bundles.
If you have already had your DockStar and WideMail plugins disabled by opening Mail.app you’ll need to look in ~/Library/Mail (or /Library/Mail if you installed globally) and move the files back to the active bundles directory. They’ll typically be in Bundles (Disabled), so quit Mail, find them, and move them back into the proper directory.

Open the Terminal.app file and enter the following:

If you have a local installation:

defaults write ~/Library/Mail/Bundles/DockStar.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "E71BD599-351A-42C5-9B63-EA5C47F7CE8E"
defaults write ~/Library/Mail/Bundles/DockStar.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "B842F7D0-4D81-4DDF-A672-129CA5B32D57"

If you have a local global installation:

defaults write /Library/Mail/Bundles/WideMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "E71BD599-351A-42C5-9B63-EA5C47F7CE8E"
defaults write /Library/Mail/Bundles/WideMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "B842F7D0-4D81-4DDF-A672-129CA5B32D57"

Launch Mail. Enjoy.

This patch should work with most any Apple Mail plugins that are disabled by the new version of Mail. That being said, use it with caution. It’s my fault if you blow it up.

Special thanks to the folks at langui.sh for pointing me in the right direction.