Update 2: Fix for 10.6.7 and Mail 4.5
Update: Fix for 10.6.5 and Mail 4.4

Another OS X release, another broken GrowlMail bundle. I did a post just like this for 10.6.2. Check it out if you want more background on why this occurs.

Easy Fix

Download a pre-patched GrowlMail.bundle and drop it in your ~/Library/Mail/Bundles/ directory¹. If you want it available to multiple users on your system, use /Library/Mail/Bundles/.

Download patched bundle

If you use this method you’re all set; no need to use the command line solution below.

Add New UUIDs to SupportedPluginCompatibilityUUIDs

If you have already had your 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.

If you have a local installation:

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

Global installation:

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

Update 3: Fix for 10.6.7 and Mail 4.5
Update 2: Fix for 10.6.5 and Mail 4.4
Update: Fix for 10.6.4 and Mail 4.3

Lately Apple has been revving the version number (and plugin compatibility UUID) of Mail.app with every version of 10.6. This breaks bundles like GrowlMail even when they are still compatible. The easy fix (although not necessarily the best if it turns out an update is required!) is to run a few commands in Terminal to add the new UUIDs¹ to the SupportedPluginCompatibilityUUID key in the Info.plist.²

If you have already had your 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.

Once you’ve gotten the files moved into the proper location use either the local installation or global installation commands below (depending on where you found your bundles). To run them, simply copy/paste them into a Terminal window.³

For GrowlMail (assuming local installation)

defaults write ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "2F0CF6F9-35BA-4812-9CB2-155C0FDB9B0F"
defaults write ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "0CB5F2A0-A173-4809-86E3-9317261F1745"

For GrowlMail (global installation)

defaults write /Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "2F0CF6F9-35BA-4812-9CB2-155C0FDB9B0F"
defaults write /Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add "0CB5F2A0-A173-4809-86E3-9317261F1745"

Update: Letterbox has been updated, but the above instructions can be adapted for any future OS update that breaks compatibility.

Update 2: You can download a pre-patched copy if you don’t want to follow the above instructions. Just unzip it and drop it in your bundles directory.

Update 3: GrowlMail 1.2.1 has been released, which fixes 10.6.2 compatibility. This issue will likely occur again with 10.6.3 though!