Building Backend by using ASP.NET Web API

Here is my testing problem.

using (MailMessage mail = new MailMessage("sendermail", "recievermail"))
{
mail.Subject = "Sending Email Using Asp.Net & C#";
mail.Body = "<h1>In this article you will learn how to send a email using Asp.Net & C#</h1>";
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.UseDefaultCredentials = false;
smtp.EnableSsl = true;
NetworkCredential NetworkCred = new NetworkCredential("sendermail", "XXXXXXXXX");
smtp.Credentials = NetworkCred;
smtp.Send(mail);
}

Please write your bid with answer of this question.
What is this code?
Why is this error happened?

ERROR: "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required. Learn more at"

I will give my whole project to right candidate.