Read the error message:
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required
You are accessing a server that requires a login before you can get at it, or that requires a secure HTTPS connection rather than an HTTP cone.
We have no idea what server, what you are trying to access, how you are trying to access it, or what credential site wants or can accept.
So start by looking at the site, your code, and maybe contact the site manager and ask him - there is nothing we can do!