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/. [...]

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 [...]