'If L{smtp.SMTPClient.sendError} is called with an L{SMTPClientError}
which is fatal, it disconnects its transport without writing anything
more to it.'

def test_sendFatalError(self):

    client = smtp.SMTPClient(None)
    transport = StringTransport()
    client.makeConnection(transport)
    client.sendError(smtp.SMTPClientError(123, 'foo', isFatal=True))
    self.assertEqual(transport.value(), '')
    self.assertTrue(transport.disconnecting)
