Why is my Dockerized Flask app timing out on mail.send? - python-3.x

Question

I'm trying to send mail from my Flask app that is served by Gunicorn & Nginx. Connection to Flask app times out when using mail.send() from container, works fine when running locally with werkzeug's development server.
I tried changing gunicorn worker classes to 'gevent' from 'sync', but the problem persists.
app.py
from flask import Flask
from werkzeug.middleware.proxy_fix import ProxyFix
from flask_sqlalchemy import SQLAlchemy
from flask_mail import Mail
app = Flask(__name__, instance_relative_config=True)
app.config.from_pyfile('config.py')
app.wsgi_app = ProxyFix(app.wsgi_app)
db = SQLAlchemy(app)
mail = Mail(app)
import views
views.py
from app import app, mail
from flask_mail import Message
import logging
#app.route('/')
def hello_world():
email_msg = Message("Subject", sender="mail#mail.com", recipients=["someone#mail.com"])
email_msg.body = f"Hello!"
logging.error("Starting email send /request")
mail.send(email_msg)
return 'Hello World!'
wsgi.py
from app import app
import logging
log = logging.getLogger('werkzeug')
log.setLevel(logging.ERROR)
if __name__ == "__main__":
app.run()#host='0.0.0.0')
Dockerfile
...
CMD ["gunicorn", "-b", "0.0.0.0:8080", "--worker-class=gevent", "--workers=9","wsgi:app"]
I expect the email to be sent, but instead it just times-out.
Gunicorn output:
gbcodes | ERROR:root:Starting email send /request
gbcodes | ERROR:app:Exception on / [GET]
gbcodes | Traceback (most recent call last):
gbcodes | File "/usr/lib/python3.6/site-packages/flask/app.py", line 2446, in wsgi_app
gbcodes | response = self.full_dispatch_request()
gbcodes | File "/usr/lib/python3.6/site-packages/flask/app.py", line 1951, in full_dispatch_request
gbcodes | rv = self.handle_user_exception(e)
gbcodes | File "/usr/lib/python3.6/site-packages/flask/app.py", line 1820, in handle_user_exception
gbcodes | reraise(exc_type, exc_value, tb)
gbcodes | File "/usr/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
gbcodes | raise value
gbcodes | File "/usr/lib/python3.6/site-packages/flask/app.py", line 1949, in full_dispatch_request
gbcodes | rv = self.dispatch_request()
gbcodes | File "/usr/lib/python3.6/site-packages/flask/app.py", line 1935, in dispatch_request
gbcodes | return self.view_functions[rule.endpoint](**req.view_args)
gbcodes | File "/app/views.py", line 13, in hello_world
gbcodes | mail.send(email_msg)
gbcodes | File "/usr/lib/python3.6/site-packages/flask_mail.py", line 491, in send
gbcodes | with self.connect() as connection:
gbcodes | File "/usr/lib/python3.6/site-packages/flask_mail.py", line 144, in __enter__
gbcodes | self.host = self.configure_host()
gbcodes | File "/usr/lib/python3.6/site-packages/flask_mail.py", line 156, in configure_host
gbcodes | host = smtplib.SMTP_SSL(self.mail.server, self.mail.port)
gbcodes | File "/usr/lib/python3.6/smtplib.py", line 1031, in __init__
gbcodes | source_address)
gbcodes | File "/usr/lib/python3.6/smtplib.py", line 251, in __init__
gbcodes | (code, msg) = self.connect(host, port)
gbcodes | File "/usr/lib/python3.6/smtplib.py", line 336, in connect
gbcodes | self.sock = self._get_socket(host, port, self.timeout)
gbcodes | File "/usr/lib/python3.6/smtplib.py", line 1037, in _get_socket
gbcodes | self.source_address)
gbcodes | File "/usr/lib/python3.6/site-packages/gevent/socket.py", line 96, in create_connection
gbcodes | sock.connect(sa)
gbcodes | File "/usr/lib/python3.6/site-packages/gevent/_socket3.py", line 335, in connect
gbcodes | raise error(err, strerror(err))
gbcodes | TimeoutError: [Errno 110] Operation timed out

Answer 1

Turns out my VPS host - Scaleway is blocking SMTP. Fixed by using port 443 which the SMTP service I use also supports.