I tried adding the code below to test_smtplib.py:

    def testAUTH_LOGIN_initial_response_notok(self):
        self.serv.add_feature("AUTH LOGIN")
        smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost',
                            timeout=support.LOOPBACK_TIMEOUT)
        resp = smtp.login(sim_auth[0], sim_auth[1], initial_response_ok=False)
        self.assertEqual(resp, (235, b'Authentication Succeeded'))
        smtp.close()

and I ended up with:

======================================================================
ERROR: testAUTH_LOGIN_initial_response_notok (test.test_smtplib.SMTPSimTests)
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Traceback (most recent call last):
  File "/home/pepoluan/projects/cpython/Lib/test/test_smtplib.py", line 1065, in testAUTH_LOGIN_initial_response_notok
    resp = smtp.login(sim_auth[0], sim_auth[1], initial_response_ok=False)
  File "/home/pepoluan/projects/cpython/Lib/smtplib.py", line 738, in login
    raise last_exception
  File "/home/pepoluan/projects/cpython/Lib/smtplib.py", line 727, in login
    (code, resp) = self.auth(
  File "/home/pepoluan/projects/cpython/Lib/smtplib.py", line 650, in auth
    raise SMTPAuthenticationError(code, resp)
smtplib.SMTPAuthenticationError: (451, b'Internal confusion')

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