Failed to send mail using smtp

See more:

ASP.NET

I am using smtpclient to send a mail.Here is my code.
while sending it will show an error like
Error:
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.


can any one tell what is the problem to sending mail?

public int sendmail(string name, string pwd,string email)
    {
        int i = 0;
        MailMessage ms = new MailMessage();
        ms.From = new MailAddress(@"ValidEmailAddress");
        ms.To.Add(email);
        ms.Subject = "Username and Password";
        ms.SubjectEncoding = System.Text.Encoding.UTF8;
        ms.IsBodyHtml = true;
        ms.Body = "UserName='" + name + "' and Password='" + pwd + "'";
        ms.BodyEncoding = System.Text.Encoding.UTF8;
        ms.Priority = MailPriority.High;
        SmtpClient clt = new SmtpClient();
        clt.Credentials = new System.Net.NetworkCredential("ValidEmailAddress", "ValidPassword");
        clt.Port = 587;
        clt.Host = "ValidHost";
        clt.EnableSsl = true;
        try

        {
            clt.Send(ms); 
            ms.Dispose();
            i = 1;
        }
        catch(Exception err) 
        {
 
        }
        return i;

[Edit] Reiss - removed user details [/Edit]