def testESMTPGreetingExtended(self):
        """
        Test that the string "ESMTP" does appear in the ESMTP server's
        greeting since L{smtp.ESMTP} does support the SMTP extensions which
        that advertises to the client.
        """
        s = smtp.ESMTP()
        t = StringTransport()
        s.makeConnection(t)
        s.connectionLost(error.ConnectionDone())
        self.assertIn(b"ESMTP", t.value())

        Test that the string "ESMTP" does appear in the ESMTP server's
        greeting since L{smtp.ESMTP} does support the SMTP extensions which
        that advertises to the client.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_smtp.py