Tagging for Mail.app Harness the full potential of spotlight right inside Mail. app. Add custom keywords to mail messages, then create smart folders for better message management. Streamline your email workflow.
Mail is missing the major capability of using labels to categorize messages. Meta provides a way to do this so you can organize beyond just sorting into mailboxes and flagging
Cons
Not well integrated into Mail - you have to use Mail's search to find your meta tags. It appears there is no way to delete tags you don't want to use and no way to edit those you are using. The trial period mechanism is obnoxious.