I once got the following exception raised by my Flask app when attempting to send an email using Flask-Mail.

STARTTLS extension not supported by server

Fortunately, I also had access to the email server. The email server was a Postfix / Dovecot email server. I mistakenly shutdown the AWS EC2 Instance running the Postfix / Dovecot email server and I found that Postfix was not running, thus the solution was oh so easy. I just had to start and enable Postfix on boot! Keep it simple, check the easy stuff first. Is the email service running!

~]$ sudo systemctl status postfix
● postfix.service - Postfix Mail Transport Agent
   Loaded: loaded (/usr/lib/systemd/system/postfix.service; disabled; vendor preset: disabled)
   Active: inactive (dead)