[MM3-users] "Must issue a STARTTLS command first" issue during social login 'send_email_confirmation'

Ubuntu 20.04 mailman3 3.2.2-1 postfix 3.4.7-2

/etc/mailman3/mailman.cfg

[mta]
incoming: mailman.mta.postfix.LMTP
outgoing: mailman.mta.deliver.deliver
smtp_host: localhost
smtp_port: 25
smtp_user: user
smtp_pass: 'password'
smtp_secure_mode: SecureMode.STARTTLS
lmtp_host: 127.0.0.1
lmtp_port: 9024
configuration: python:mailman.config.postfix

The smtp_secure_mode value is based on https://mailman.readthedocs.io/en/latest/src/mailman/mta/docs/connection.htm.... I am able to send emails using a mail client located on a remote device with the same user/password credentials. Yet, trying to perform a social login leads to:

ERROR 2019-12-17 11:44:00,414 211904 django.request Internal Server Error: /accounts/github/login/callback/
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/allauth/socialaccount/providers/oauth2/views.py", line 73, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/allauth/socialaccount/providers/oauth2/views.py", line 143, in dispatch
    return complete_social_login(request, login)
  File "/usr/lib/python3/dist-packages/allauth/socialaccount/helpers.py", line 152, in complete_social_login
    return _complete_social_login(request, sociallogin)
  File "/usr/lib/python3/dist-packages/allauth/socialaccount/helpers.py", line 167, in _complete_social_login
    ret = _login_social_account(request, sociallogin)
  File "/usr/lib/python3/dist-packages/allauth/socialaccount/helpers.py", line 55, in _login_social_account
    signal_kwargs={"sociallogin": sociallogin})
  File "/usr/lib/python3/dist-packages/allauth/account/utils.py", line 144, in perform_login
    send_email_confirmation(request, user, signup=signup)
  File "/usr/lib/python3/dist-packages/allauth/account/utils.py", line 312, in send_email_confirmation
    signup=signup)
  File "/usr/lib/python3/dist-packages/allauth/account/models.py", line 60, in send_confirmation
    confirmation.send(request, signup=signup)
  File "/usr/lib/python3/dist-packages/allauth/account/models.py", line 166, in send
    get_adapter(request).send_confirmation_mail(request, self, signup)
  File "/usr/lib/python3/dist-packages/allauth/account/adapter.py", line 450, in send_confirmation_mail
    ctx)
  File "/usr/lib/python3/dist-packages/allauth/account/adapter.py", line 137, in send_mail
    msg.send()
  File "/usr/lib/python3/dist-packages/django/core/mail/message.py", line 291, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "/usr/lib/python3/dist-packages/django/core/mail/backends/smtp.py", line 110, in send_messages
    sent = self._send(message)
  File "/usr/lib/python3/dist-packages/django/core/mail/backends/smtp.py", line 126, in _send
    self.connection.sendmail(from_email, recipients, message.as_bytes(linesep='\r\n'))
  File "/usr/lib/python3.7/smtplib.py", line 867, in sendmail
    raise SMTPSenderRefused(code, resp, from_addr)
smtplib.SMTPSenderRefused: (530, b'5.7.0 Must issue a STARTTLS command first', 'user@mysite')

Any suggestion?