Error when sending email from mail server other than GMAIL

Your are using host server mail.cliffsupport.com while you are using settings of GMAIL. Try the following. If it still does not work. Contact your hosting company or your network administrator to give you mail settings

MailMessage mm = new MailMessage();
        mm.From = new MailAddress(ConfigurationManager.AppSettings["Email"].ToString());
        mm.To.Add(new MailAddress("gopikshnan@gmail.com"));
        mm.Subject = Subject;
        mm.Body = Body;
        mm.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        //smtp.Host = "smtp.gmail.com";
        smtp.Host = "mail.cliffsupport.com";
        NetworkCredential nwkCredential = new NetworkCredential();
        nwkCredential.UserName = "gopi@cliffsupport.com";
        nwkCredential.Password = "mypassord";
        smtp.UseDefaultCredentials = true;
        smtp.Credentials = nwkCredential;
        smtp.Port = 25;
        smtp.Send(mm);