fail to send email

Hi I am a beginner on Mautic and encountered issues in setting email configurations.
I configured the SMTP server and sent test email successfully. When I sent emails to our contact, it was rejected.
The error message says: SMPT server not authenticated.

I tried multiple SMTP servers including gmail when configuring senders’s email, they all passed tests, but none of them was able to successfully send emails to real contacts. Does anyone know why?