Why Am I Getting a 'The Remote Certificate Is Invalid' Error When Sending Emails via Gmail's SMTP Server in C#?

Troubleshooting the "Remote Certificate Is Invalid" Error in Gmail SMTP

Your C# application might fail to send emails via Gmail's SMTP server, displaying the error: "The remote certificate is invalid according to the validation procedure." This happens when your application's secure connection (TLS/SSL) can't verify Gmail's certificate.

Temporary Fix (Not Recommended for Production):

A quick, but highly insecure, solution is to temporarily disable certificate validation. Use this ONLY for testing and debugging. Never deploy this to a production environment.

[Obsolete("Do not use this in Production code!!!", true)]
static void DisableCertificateValidation() //Renamed for clarity
{
    // Disabling certificate validation exposes your application to man-in-the-middle attacks.
    // Your encrypted messages could be intercepted and read.
    // See: https://stackoverflow.com/a/14907718/740639
    ServicePointManager.ServerCertificateValidationCallback =
        (s, certificate, chain, sslPolicyErrors) => true;
}

Secure Solution:

The proper approach involves verifying Gmail's SMTP certificate. Ensure the certificate is:

• Valid: Not expired.
• Trusted: Signed by a trusted Certificate Authority (CA).
• Correct: Belongs to the expected domain (e.g., smtp.gmail.com).

Understanding the Stack Trace:

The stack trace (not provided in this revised response, as it wasn't included in the prompt's output) details the method calls leading to the error. A failure at CheckCompletionBeforeNextReceive usually indicates a problem with certificate validation or encryption during the SSL/TLS handshake. Analyzing the stack trace helps pinpoint the exact location of the failure within your code.

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