Every time Apple releases a new version of Mail (which it did just last week), users howl because it breaks Mail add-ins like GrowlMail, Mail ActOn, and others. We then have to wait while the developers tweak those add-ins to work with the new Mail. Sometimes that can take a while. As several Hints readers have discovered, you can update the add-ins yourself, so they work with the new Mail.
The crux of the problem is that bundles like GrowlMail are configured to work with specific versions of Mail.app and the Message.framework; those versions are identified by their Universally Unique Identifiers (UUIDs). To get the old add-ins working with the new Mail, you have to tell those bundles what the new UUIDs are.
The first thing you need to do is find out what those UUIDs are. To do so, enter the following two commands, one after the other, in the Terminal's command line:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID
That should return two strings, in this case
857A142A-AB81-4D99-BECC-D1B55A86D94E. Those are the new UUIDs for Mail 10.6.5, but that same technique should work for future updates as well.
In the case of GrowlMail, you then need to add those strings to the info.plist file found in GrowlMail.mailbundle. That bundle would have originally been installed in ~/Library/Mail/Bundles/ or /Library/Mail/Bundles/. But chances are that, when you updated to the new version of Mail, GrowlMail.mailbundle was moved to a disabled bundles folder. If that's happened, you'll need to drag the bundle from the disabled folder to the original /Bundles.
Once you find the file, quit Mail, drag the bundle in the active /Bundles folder, and open its info.plist file in a plain-text editor. (In some cases, you'll have to Control-click on the bundle file itself then select Show Package Contents from the contextual menu to find the info.plist file.) Then, in info.plist find the string
SupportedPluginCompatibilityUUIDs. Below that, you should see a list of strings that look like the UUIDs you found for Mail and Message.framework. Following the same format as those other strings, add the UUIDs for the new version of Mail. Save the file, restart Mail, and your bundle should be back in business.