Fix Gmail SMTP Auth Error

I have an ASP.NET web-service application running on a dedicated Windows server. Email is sent through Gmail’s SMTP relay, but every attempt returns “5.7.0 Authentication Required.” The credentials in web.config are correct, so the problem sits somewhere in the SMTP section (SSL/TLS settings, port choice, Google account security, or Windows-level restrictions). I am not certain whether the Google account is already configured to allow “Less secure apps,” so that may need checking or an App Password approach if 2-Step Verification is active.

Your task is to review the current web.config, server network rules, and Gmail account settings, apply the proper authentication method, and prove the fix by sending a successful test email from the application. A brief outline of the steps you took and any changes made should be supplied once the issue is resolved.