Warning: Do not use this in production code!

As a workaround, you can switch off certificate validation. Only ever do this to obtain confirmation that the error is being thrown because of a bad certificate.

Call this method before you call smtpclient.Send():

[Obsolete("Do not use this in Production code!!!",true)]
static void NEVER_EAT_POISON_Disable_CertificateValidation()
{
    // Disabling certificate validation can expose you to a man-in-the-middle attack
    // which may allow your encrypted message to be read by an attacker
    // https://stackoverflow.com/a/14907718/740639
    ServicePointManager.ServerCertificateValidationCallback =
        delegate (
            object s,
            X509Certificate certificate,
            X509Chain chain,
            SslPolicyErrors sslPolicyErrors
        ) {
            return true;
        };
}