public ActionResult SendMail(MailWriteLetterViewModel viewModel)
        {
            List<CustomerDataModel> customers = (List<CustomerDataModel>)TempData["CustomersToSend"];
            List<string> emails = (from c in customers
                                   select c.Email).ToList<string>();

            // send mails in second thread
            Thread thread = new Thread(delegate() { Mailer.SendMail(emails, viewModel.Title, viewModel.Text); });
            thread.Start();

            return View(new MailSendViewModel());
        }
        public ActionResult WriteLetter(FormCollection collection)
        {
            // select id of every checked customer
            int[] checkedCustomersId = collection.GetValues("id_customer").Select(n => Convert.ToInt32(n)).ToArray();

            CustomerService cService = new CustomerService();

            MailWriteLetterViewModel viewModel = new MailWriteLetterViewModel();
            viewModel.CustomersToSend = cService.GetCustomers(c => checkedCustomersId.Contains(c.Id));
            TempData["CustomersToSend"] = viewModel.CustomersToSend;

            return View(viewModel);
        }