The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required.

Error:

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required.

Resolution:

Go to you email account settings at https://myaccount.google.com/

Security settings required by G Suite.

1. A strong password. I create a new strong password. Confirmed.
2. 2-factor auth is off. Confirmed.
3. Access to less secure apps is enabled. Here is screenshot of my settings, the first is the admin console enabling it for users, the second is the user enabling it:

Reference:

https://superuser.com/questions/1582593/the-smtp-server-requires-a-secure-connection-or-the-client-was-not-authenticated