docs: def _build_message(self, to, text, subject=None, mtype=None, unsubscribe_url=None):
          """Constructs a MIME message from message and dispatch models."""
          # TODO Maybe file attachments handling through `files` message_model context var.
  
          if subject is None:
              subject = u'%s' % _('No Subject')
  
          if mtype == 'html':
              msg = self.mime_multipart()
              text_part = self.mime_multipart('alternative')
              text_part.attach(self.mime_text(strip_tags(text), _charset='utf-8'))
              text_part.attach(self.mime_text(text, 'html', _charset='utf-8'))
              msg.attach(text_part)
  
          else:
              msg = self.mime_text(text, _charset='utf-8')
  
          msg['From'] = self.from_email
          msg['To'] = to
          msg['Subject'] = subject
  
          if unsubscribe_url:
              msg['List-Unsubscribe'] = '<%s>' % unsubscribe_url
  
          return msg