The Gentoo Refleaks 3.7 buildbot is reporting that test_smtplib may be leaving with dangling threads:

https://buildbot.python.org/all/#builders/114/builds/179

FAILED (errors=7)
Warning -- threading._dangling was modified by test_smtplib
  Before: <_weakrefset.WeakSet object at 0xb6918bcc>
  After:  <_weakrefset.WeakSet object at 0xb68e761c> 
test test_smtplib failed
1 test failed again:
    test_smtplib
== Tests result: FAILURE then FAILURE ==

New failure on AMD64 Windows8.1 Refleaks 3.6:

https://buildbot.python.org/all/#builders/64/builds/301

Ah, sorry.  we just modernized test_smtplib's __main__ section, and I think we lost the thread cleanup code that used to be run around the test suite.  We need to add the thread cleanup decorators.

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

The TypeError is because the test is misusing the assertRaises API:

         self.assertRaises(smtplib.SMTPNotSupportedError,
                          smtp.send_message(msg))

should be 

         self.assertRaises(smtplib.SMTPNotSupportedError,
                          smtp.send_message, msg)

That's not the code I thought I merged.  I'll have to take a look at what actually got merged.

That was from commit 48ed88a93bb0bbeaae9a4cfaa533e4edf13bcb51

Oh, sorry, my bad. That was precisely what commit 48ed88a93bb0bbeaae9a4cfaa533e4edf13bcb51 changed. The problem with running with -R still happens:

======================================================================
ERROR: test_name_field_not_included_in_envelop_addresses (test.test_smtplib.SMTPSimTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/pgalindo3/cpython/Lib/test/test_smtplib.py", line 1100, in test_name_field_not_included_in_envelop_addresses
    self.assertDictEqual(smtp.send_message(message), {})
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 967, in send_message
    rcpt_options)
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 861, in sendmail
    (code, resp) = self.mail(from_addr, esmtp_opts)
  File "/Users/pgalindo3/cpython/Lib/smtplib.py", line 532, in mail
    'SMTPUTF8 not supported by server')
smtplib.SMTPNotSupportedError: SMTPUTF8 not supported by server

----------------------------------------------------------------------
Ran 60 tests in 1.365s

FAILED (errors=7)
Warning -- threading._dangling was modified by test_smtplib
  Before: <_weakrefset.WeakSet object at 0x10f445e90>
  After:  <_weakrefset.WeakSet object at 0x10fe75ad0>
test test_smtplib failed
test_smtplib failed

== Tests result: FAILURE ==

1 test failed:
    test_smtplib

Total duration: 2 sec 920 ms
Tests result: FAILURE

That's due to another bug, which will get fixed by #32814, unless you'd like to generate a PR with the this fix in it (fixing the mutable default for smtp_options).

This is just to note that I'm investigating this issue as part of the CPython sprint in EuroPython.

See also issue32657.

New changeset d5fbe9b1a3d65ceeb9159c5ba999ee966a945f76 by Pablo Galindo (Pablo Aguiar) in branch 'master':
bpo-34246: Use no mutable default args in smtplib (GH-8554)
https://github.com/python/cpython/commit/d5fbe9b1a3d65ceeb9159c5ba999ee966a945f76

New changeset 9835696ec4c57a9a30f1c11cfb4c5d3e121bf97c by Miss Islington (bot) in branch '3.7':
bpo-34246: Use no mutable default args in smtplib (GH-8554)
https://github.com/python/cpython/commit/9835696ec4c57a9a30f1c11cfb4c5d3e121bf97c

New changeset 5b7a2cb5caeb7df68e637f45a98632cbc84a51bf by Pablo Galindo in branch 'master':
bpo-34246: Make sure test_smtplib always cleans resources when finished (GH-9108)
https://github.com/python/cpython/commit/5b7a2cb5caeb7df68e637f45a98632cbc84a51bf

New changeset eb6ab73f93c8b883a8d75a83560e2b4c59170d95 by Pablo Galindo in branch '3.6':
[3.6] bpo-34246: Use no mutable default args in smtplib (GH-8554) (#9112)
https://github.com/python/cpython/commit/eb6ab73f93c8b883a8d75a83560e2b4c59170d95