Unable to send email using C#(server response: 5.7.0 Authentication Required)

Question

I am trying to send emails from my gmail account for testing. The code I am using is pasted below. I have checked the paramters that are being sent and they all are correct. Still I am facing exception "Error: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required". I am not sure what am I doing wrong here!

public string MssSendEMail(string ssToAddress, string ssFromAddress, string ssSubject, string ssMessageBody, int ssSMTPPort, string ssSMTPHost, bool ssIsSSLEnabled, bool ssIsUsingDefaultCredentials, string ssUserName, string ssPassword)
        {
            string ssResult = "";
            try
            {
                MailMessage message = new MailMessage();
                SmtpClient smtp = new SmtpClient();
                message.From = new MailAddress(ssFromAddress);
                message.To.Add(new MailAddress(ssToAddress));
                message.Subject = ssSubject;
                message.IsBodyHtml = true;  
                message.Body = ssMessageBody;
                smtp.Port = ssSMTPPort; //587
                smtp.Host = ssSMTPHost; //smtp.gmail.com
                smtp.EnableSsl = true;
                smtp.UseDefaultCredentials = false;
                smtp.Credentials = new NetworkCredential(ssUserName, ssPassword);
                smtp.Send(message);
                ssResult = "EMail Sent!";
            }
            catch (Exception ex)
            {
                ssResult = "Error: " + ex.Message;
            }
            return ssResult;
        }