Robin Bonathan got it working again. Great detective work!

I got growl mail to work again afterwards.

You need to change 1 info.plist file. (use textedit to modify)

With mail off.

Go to your user login directory and follow this path:-

~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/
Info.plist

open up info.plist

and in the area under <key>SupportedPluginCompatibilityUUIDs</key>
Then add:
<string>9049EF7D-5873-4F54-A447-51D722009310</string>
<string>1C58722D-AFBD-464E-81BB-0E05C108BE06</string>

save the file and now Growl is once again working…

via GrowlMail not working in Mail 4.5: Apple Support Communities.