Constructs the actual underlying message with provided values

def _setup_message(self):
        """Constructs the actual underlying message with provided values"""
        if self.content == 'html':
            self._message = MIMEMultipart('alternative')
            part = MIMEText(self.body, 'html', 'UTF-8')
        else:
            self._message = MIMEMultipart()
            part = MIMEText(self.body, 'plain', 'UTF-8')
        self._message.preamble = 'Multipart massage.\n'
        self._message.attach(part)
        self._message['From'] = self.sender
        self._message['To'] = COMMASPACE.join(self.to)
        if self.cc:
            self._message['Cc'] = COMMASPACE.join(self.cc)
        self._message['Date'] = formatdate(localtime=True)
        self._message['Subject'] = self.subject
        for part in self._parts:
            self._message.attach(part)