Problem Statement:  

Getting error while sending Test email "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 SMTP; Client was not authenticated to send".

Propose Solution: 

Please check the below information to solve the issue.

1: Make sure the Server Port number is correct.

2: Make sure the User Name and User Password is correct.

3: Make sure the email is allowed for less secure app access.

For Gmail, please refer below link: -

https://myaccount.google.com/lesssecureapps