This is my code for 
Sending Email to Multiple Recipients it throws an exception
 The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.
public static void SendEmail(string ToEmail, string subject, string message)
 {
 //Reading sender Email credential from web.config file
 HostAdd = ConfigurationManager.AppSettings["Host"].ToString();
 FromEmailid = ConfigurationManager.AppSettings["FromMail"].ToString();
 Pass = ConfigurationManager.AppSettings["Password"].ToString();
 //creating the object of MailMessage
 MailMessage mailMessage = new MailMessage();
 mailMessage.From = new MailAddress(FromEmailid); //From Email Id
 mailMessage.Subject = subject; //Subject of Email
 mailMessage.Body = message; //body or message of Email
 mailMessage.IsBodyHtml = true;
 //Adding Multiple recipient email id logic
 string[] Multi = ToEmail.Split(','); //spiliting input Email id string with comma(,)
 foreach (string Multiemailid in Multi)
 {
 mailMessage.To.Add(new MailAddress(Multiemailid)); //adding multi reciver's Email Id
 //mailMessage.To.Add("
[email protected]");
 }
 SmtpClient smtp = new SmtpClient(); // creating object of smptpclient
 smtp.Host = HostAdd; //host of emailaddress for example smtp.gmail.com etc
 //network and security related credentials
 smtp.EnableSsl = true;
 NetworkCredential NetworkCred = new NetworkCredential();
 NetworkCred.UserName = mailMessage.From.Address;
 NetworkCred.Password = Pass;
 // smtp.UseDefaultCredentials = true;
 smtp.Credentials = NetworkCred;
 smtp.Port = 25;
 smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
 smtp.Send(mailMessage); //sending Email 
 }