Why Is My Gmail SMTP Connection Failing with an 'Invalid Certificate' Error?

Troubleshooting Gmail SMTP's "Invalid Certificate" Error:

Sending emails via Gmail's SMTP server might trigger the error "The remote certificate is invalid according to the validation procedure." This article explains the cause and offers a temporary solution.

The problem lies in the certificate validation process. Gmail's SMTP server uses a security certificate to verify the authenticity of email data. If your client's validation process rejects the server's certificate, this error appears.

Temporary Solution (For Debugging Only):

To temporarily bypass certificate validation (strongly discouraged for production environments due to significant security risks), use this method before initiating smtpclient.Send():

<code class="language-csharp">[Obsolete("Never use this in production code!", true)]
static void DisableCertificateValidation()
{
    // Disabling certificate validation exposes your application to man-in-the-middle attacks,
    // allowing attackers to potentially intercept and read your encrypted messages.
    // See: https://stackoverflow.com/a/14907718/740639
    ServicePointManager.ServerCertificateValidationCallback =
        (s, certificate, chain, sslPolicyErrors) => true;
}</code>

This code tells the .NET Framework to accept any server certificate. Repeat: Only use this for diagnostic purposes. Never deploy code with certificate validation disabled. Implementing this solution in a production setting severely weakens your application's security.

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