Problem to send a mail

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C#3.5

I am getting following error when i tried to send mail through my asp.net c# code

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first. y2sm48568819pfi.39 - gsmtp

What I have tried:

I am getting following error when i tried to send mail through my asp.net c# code

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first. y2sm48568819pfi.39 - gsmtp

my code is:

MailMessage mm = new MailMessage("xyz@gmail.com", "xyz@gmail.com");
           mm.Subject = "Password Recovery";
           mm.Body = string.Format("Hi ,<br /><br />Your password is .<br /><br />Thank You.");
           mm.IsBodyHtml = true;
           SmtpClient smtp = new SmtpClient();
           smtp.Host = "smtp.gmail.com";
           smtp.EnableSsl = true;
           NetworkCredential NetworkCred = new NetworkCredential();
           NetworkCred.UserName = "xyz@gmail.com";
           NetworkCred.Password = "xyz";
           smtp.UseDefaultCredentials = true;
           smtp.Credentials = NetworkCred;
           smtp.Port = 587;
           smtp.Send(mm);