Email for RapidWeaver is designed to export HTML email that can be used in online newsletter/email services such as MailChimp, MadMimi, AWeber, Sendy, etc. It does not currently support creation of email templates to be used inside Mail.app.
Its powered by Foundation for Emails from my good friends at Zurb. Its a powerful framework that has been tested against all major email clients. It even support responsive layouts! The included demo file has 15 wonderful templates to get you started as well.