def extensions(self):
        """
        SMTP service extensions

        @return: the SMTP service extensions that are supported.
        @rtype: L{dict} with L{bytes} keys and a value of either L{None} or a
            L{list} of L{bytes}.
        """
        ext = {b'AUTH': _keys(self.challengers)}
        if self.canStartTLS and not self.startedTLS:
            ext[b'STARTTLS'] = None
        return ext

        SMTP service extensions

        @return: the SMTP service extensions that are supported.
        @rtype: L{dict} with L{bytes} keys and a value of either L{None} or a
            L{list} of L{bytes}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/smtp.py