Local host : 28642 says object reference not set to an instance of an object

           EmailSubj = "Complaint Filed  ";
           EmailMsg  = Convert.ToString(txt_Comp_memo.Text);

           string[] emails = new string[] { txt_Email.Text, txt_Zemail.Text, txt_Bemail.Text };
           foreach (var email in emails)
           {
               if (!string.IsNullOrWhiteSpace(email))
                   SendEmail(email, EmailSubj, EmailMsg);



private void SendEmail(String ToEmail, String Subj, string Message)
   {
       try
       {
           //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 = Subj; //Subject of Email
           mailMessage.Body = Message; //body or message of Email
           mailMessage.IsBodyHtml = true;
           mailMessage.To.Add(new MailAddress(ToEmail)); //reciver's Email Id

           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 = 587;
           smtp.Send(mailMessage); //sending Email

           string error = "Message Sent successfully. Thank you..!!";

           ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('" + error + "');", true);

           txt_Comp_memo.Text = " ";
           txt_Email.Text = " ";
           txt_Zemail.Text = " ";
           txt_Bemail.Text = " ";


       }
       catch (Exception ex)
       {
           string error = ex.Message;
           ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('" + error + "');", true);
       }

       finally
       {

       }

   }

           }