Python/Flask Bug SMTP authentication error

I have a python/flask app deployed on my CentOS VPS which receives orders from customers. It also connects to Stripe to take payments. The problem is that our "order summary" emails are not being received. I believe this has to do with our SMTP authentication as you can see the error received when customer place an order attached below.

Your job is to connect to my computer via Teamviewer tonight and help me find the issue then fix it.

Thank you