How to remove GrowlMail (on Mavericks)?
Close Mail.app

Open a Terminal and enter chflags nohidden ~/Library.

Now you can navigate with your Finder to the following directories and delete the plugin GrowlMail.mailbundle:


~/Library/Mail/Bundles/GrowlMail.mailbundle
/Library/Mail/Bundles/GrowlMail.mailbundle 


When starting Mail.app now, you shouldn't get any error messages related to GrowlMail.