mail_to = str(self.request.POST.get('to'))
mail_from = str(self.request.POST.get('from'))
mail_subject = str(self.request.POST.get('subject'))
mail_body = str(self.request.POST.get('body'))
mail.send_mail(mail_from, mail_to, mail_subject, mail_body, html=mail_html_body
    )
Google App Engine: как отправить html с помощью send_mail