"The remote certificate is invalid according to the validation procedure." using Gmail SMTP server

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 throw 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;
            };
    }

The link here solved my problem.

http://brainof-dave.blogspot.com.au/2008/08/remote-certificate-is-invalid-according.html

I went to url of the web service (on the server that had the issue), clicked on the little security icon in IE, which brought up the certificate. I then clicked on the Details tab, clicked the Copy To File button, which allowed me to export the certifcate as a .cer file. Once I had the certificate locally, I was able to import it into the certificate store on the server using the below instructions.

Start a new MMC. File --> Add/Remove Snap-In... Click Add... Choose Certificates and click Add. Check the "Computer Account" radio button. Click Next.

Choose the client computer in the next screen. Click Finish. Click Close. Click OK. NOW install the certificate into the Trusted Root Certification Authorities certificate store. This will allow all users to trust the certificate.

A little late to the party, but if you are looking for a solution like Yury's the following code will help you identify if the issue is related to a self-sign certificate and, if so ignore the self-sign error. You could obviously check for other SSL errors if you so desired.

The code we use (courtesy of Microsoft - http://msdn.microsoft.com/en-us/library/office/dd633677(v=exchg.80).aspx) is as follows:

  private static bool CertificateValidationCallBack(
         object sender,
         System.Security.Cryptography.X509Certificates.X509Certificate certificate,
         System.Security.Cryptography.X509Certificates.X509Chain chain,
         System.Net.Security.SslPolicyErrors sslPolicyErrors)
    {
  // If the certificate is a valid, signed certificate, return true.
  if (sslPolicyErrors == System.Net.Security.SslPolicyErrors.None)
  {
    return true;
  }

  // If there are errors in the certificate chain, look at each error to determine the cause.
  if ((sslPolicyErrors & System.Net.Security.SslPolicyErrors.RemoteCertificateChainErrors) != 0)
  {
    if (chain != null && chain.ChainStatus != null)
    {
      foreach (System.Security.Cryptography.X509Certificates.X509ChainStatus status in chain.ChainStatus)
      {
        if ((certificate.Subject == certificate.Issuer) &&
           (status.Status == System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.UntrustedRoot))
        {
          // Self-signed certificates with an untrusted root are valid. 
          continue;
        }
        else
        {
          if (status.Status != System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.NoError)
          {
            // If there are any other errors in the certificate chain, the certificate is invalid,
         // so the method returns false.
            return false;
          }
        }
      }
    }

    // When processing reaches this line, the only errors in the certificate chain are 
// untrusted root errors for self-signed certificates. These certificates are valid
// for default Exchange server installations, so return true.
    return true;
  }
  else
  {
 // In all other cases, return false.
    return false;
  }
}