core: SMTP Integration not connecting over TLS since 2024.1

The problem

Starting in 2024.1, my SMTP integration is no longer working. It worked fine in 2023.12, but since 2024.1, I have to set verify_ssl: false or I get a stack trace when I try to send an email.

What version of Home Assistant Core has the issue?

2024.1

What was the last working version of Home Assistant Core?

2023.12

What type of installation are you running?

Home Assistant OS

Integration causing the issue

SMTP

Link to integration documentation on our website

https://www.home-assistant.io/integrations/smtp/

Diagnostics information

No response

Example YAML snippet

notify:
  - name: "SMTP Notifications"
    platform: smtp
    sender: "homeassistant@example.com"
    unique_id: smtp_notifications
    recipient:
      - alex@example.com
    server: smtp.fastmail.com
    port: 465
    encryption: tls
    username: alex@example.com
    password: !secret alex_smtp_password
    sender_name: Ives Home

### Currently working

  - name: "Working SMTP Notifications"
    platform: smtp
    sender: "homeassistant@example.com"
    unique_id: smtp_notifications
    recipient:
      - alex@example.com
    server: smtp.fastmail.com
    port: 465
    encryption: tls
    username: alex@example.com
    password: !secret alex_smtp_password
    sender_name: Ives Home
    verify_ssl: false

Anything in the logs that might be useful for us?

Error on reload after removing verify_ssl: true from the config.

Logger: homeassistant.components.notify
Source: components/notify/legacy.py:94
Integration: Notifications (documentation, issues)
First occurred: 10:34:35 AM (4 occurrences)
Last logged: 10:43:29 AM

Error setting up platform smtp
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/components/notify/legacy.py", line 94, in async_setup_platform
    notify_service = await hass.async_add_executor_job(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 102, in get_service
    if mail_service.connection_is_valid():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 164, in connection_is_valid
    server = self.connect()
             ^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 143, in connect
    mail = smtplib.SMTP_SSL(
           ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 1050, in __init__
    SMTP.__init__(self, host, port, local_hostname, timeout,
  File "/usr/local/lib/python3.11/smtplib.py", line 255, in __init__
    (code, msg) = self.connect(host, port)
                  ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 341, in connect
    self.sock = self._get_socket(host, port, self.timeout)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 1057, in _get_socket
    new_socket = self.context.wrap_socket(new_socket,
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/ssl.py", line 517, in wrap_socket
    return self.sslsocket_class._create(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/ssl.py", line 1108, in _create
    self.do_handshake()
  File "/usr/local/lib/python3.11/ssl.py", line 1379, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL] unknown error (_ssl.c:1006)

Error on trying to send (only possible if I restart, since the yaml reload checks the connection)

Logger: homeassistant.components.websocket_api.http.connection
Source: components/smtp/notify.py:143
Integration: Home Assistant WebSocket API (documentation, issues)
First occurred: 10:30:39 AM (3 occurrences)
Last logged: 10:30:52 AM

[547303142848] Error handling message: Unknown error (unknown_error) Alex from 192.168.1.203 (Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:122.0) Gecko/20100101 Firefox/122.0)
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/components/websocket_api/decorators.py", line 26, in _handle_async_response
    await func(hass, connection, msg)
  File "/usr/src/homeassistant/homeassistant/components/websocket_api/commands.py", line 782, in handle_execute_script
    script_result = await script_obj.async_run(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 1587, in async_run
    return await asyncio.shield(run.async_run())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 426, in async_run
    await self._async_step(log_exceptions=False)
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 479, in _async_step
    self._handle_exception(
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 502, in _handle_exception
    raise exception
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 476, in _async_step
    await getattr(self, handler)()
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 713, in _async_call_service_step
    response_data = await self._async_run_long_action(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/helpers/script.py", line 675, in _async_run_long_action
    return long_task.result()
           ^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/core.py", line 2149, in async_call
    response_data = await coro
                    ^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/core.py", line 2186, in _execute_service
    return await target(service_call)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/notify/legacy.py", line 265, in _async_notify_message_service
    await self.async_send_message(**kwargs)
  File "/usr/src/homeassistant/homeassistant/components/notify/legacy.py", line 241, in async_send_message
    await self.hass.async_add_executor_job(
  File "/usr/local/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 223, in send_message
    return self._send_email(msg, recipients)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 227, in _send_email
    mail = self.connect()
           ^^^^^^^^^^^^^^
  File "/usr/src/homeassistant/homeassistant/components/smtp/notify.py", line 143, in connect
    mail = smtplib.SMTP_SSL(
           ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 1050, in __init__
    SMTP.__init__(self, host, port, local_hostname, timeout,
  File "/usr/local/lib/python3.11/smtplib.py", line 255, in __init__
    (code, msg) = self.connect(host, port)
                  ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 341, in connect
    self.sock = self._get_socket(host, port, self.timeout)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/smtplib.py", line 1057, in _get_socket
    new_socket = self.context.wrap_socket(new_socket,
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/ssl.py", line 517, in wrap_socket
    return self.sslsocket_class._create(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/ssl.py", line 1108, in _create
    self.do_handshake()
  File "/usr/local/lib/python3.11/ssl.py", line 1379, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL] unknown error (_ssl.c:1006)

Additional information

Originally I thought this might be related to https://github.com/home-assistant/core/issues/107369, but it seems that’s resolved.