How to create a working SMTPS server on port 465 using Python?

I'm trying to set up an SMTPS server on port 465 using Python and the aiosmtpd library. While I can connect to the server locally using openssl s_client, external mail servers (like Gmail) are unable to connect, reporting a "Connection refused" error.

Here's my current code:

import asyncio
import ssl
from aiosmtpd.controller import Controller
from aiosmtpd.handlers import Debugging

async def main():
    ssl_context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
    ssl_context.load_cert_chain(
        certfile='./certs/fullchain.pem',
        keyfile='./certs/privkey.pem'
    )

    controller = Controller(
        handler=Debugging(),
        hostname='0.0.0.0',
        port=465,
        ssl_context=ssl_context
    )

    controller.start()
    print("SMTPS server started on port 465...")

    try:
        await asyncio.sleep(3600)
    finally:
        controller.stop()

if __name__ == "__main__":
    asyncio.run(main())

Though it works fine when I disable TLS encryption and change port number to 25.

I've verified that:

1. The server starts without errors.
2. I can connect to my server using openssl s_client -connect my_domain.example.com:465.
3. The port 465 is open in my firewall.
4. The SSL certificates are valid and match the domain name.

I know that there is also SMTP with STARTTLS protocol on port 587, but in my specific use case I need exactly SMTPS on port 465.

Additional information:

Python version: 3.12.3. OS: Ubuntu 24 server

Any help or guidance would be greatly appreciated. Thank you