How to Fix the 'The Remote Certificate is Invalid' Error When Using Gmail's SMTP Server in C#?

Troubleshooting "The Remote Certificate is Invalid" Error When Using Gmail's SMTP Server in C#

C# developers using Gmail's SMTP server for email sending may encounter the error: "The remote certificate is invalid according to the validation procedure." This occurs because the security certificate validation fails. The following solution should only be used for debugging; never deploy this to a production environment.

Temporarily Disabling Certificate Validation (For Debugging ONLY)

Critical Security Warning: Disabling certificate validation exposes your application to serious security risks, including man-in-the-middle attacks where your encrypted emails could be intercepted. This should be used solely to isolate the problem, not as a permanent solution.

Step 1: Implement a Temporary Disable Function

Before calling smtpclient.Send(), execute this function:

<code class="language-csharp">[Obsolete("Do not use this in production code!!!", true)]
static void DisableCertificateValidationForDebugging()
{
    // Disabling certificate validation is extremely risky and should only be used for troubleshooting.
    // It exposes your application to man-in-the-middle attacks.
    ServicePointManager.ServerCertificateValidationCallback = (_, certificate, chain, sslPolicyErrors) => true;
}</code>

This temporarily overrides the certificate validation process. After confirming the certificate is the source of the error, immediately revert this change and address the root cause.

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