I think it may be something extra as test_send_message_error_on_non_ascii_addrs_if_no_smtputf8 is failing when you run the test with -R:

./python -W error -m test test_smtplib  -R 3:3

Warning -- threading._dangling was modified by test_smtplib
  Before: <_weakrefset.WeakSet object at 0x10ed7dd70>
  After:  <_weakrefset.WeakSet object at 0x10f619830>
test test_smtplib failed -- Traceback (most recent call last):
  File "/Users/pgalindo3/cpython/Lib/test/test_smtplib.py", line 1232, in test_send_message_error_on_non_ascii_addrs_if_no_smtputf8
    smtp.send_message(msg))
  File "/Users/pgalindo3/cpython/Lib/unittest/case.py", line 743, in assertRaises
    return context.handle('assertRaises', args, kwargs)
  File "/Users/pgalindo3/cpython/Lib/unittest/case.py", line 178, in handle
    callable_obj(*args, **kwargs)
TypeError: 'dict' object is not callable

test_smtplib failed

== Tests result: FAILURE ==

1 test failed:
    test_smtplib

Total duration: 16 sec 566 ms
Tests result: FAILURE