def test_requireTransportSecurityTLSNotOffered(self):
        """
        When C{requireTransportSecurity} is C{True} and the client is connected
        over a non-SSL transport, if the server does not offer the I{STARTTLS}
        extension, mail is not delivered.
        """
        transport = StringTransport()
        self.clientProtocol.makeConnection(transport)

        # Get the handshake out of the way
        self.clientProtocol.dataReceived(self.SERVER_GREETING)
        transport.clear()

        # Tell the client about the server's capabilities - excluding STARTTLS
        self.clientProtocol.dataReceived(
            self.EHLO_RESPONSE +
            b"250 AUTH LOGIN\r\n")

        # The client give up
        self.assertEqual(b"QUIT\r\n", transport.value())

        When C{requireTransportSecurity} is C{True} and the client is connected
        over a non-SSL transport, if the server does not offer the I{STARTTLS}
        extension, mail is not delivered.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_smtp.py