namespace EmailSender
    {
        public class SendingMail
        {
            private static List<Customer> customers = DataLayer.ListCustomers();
            public static void SendEmail(string recipients, string from, string subject, string body)
            {

            foreach (Customer customer in customers)
            {
                var mailMessage = new MailMessage();
                mailMessage.To.Add(string.Join(",", recipients));
                mailMessage.From = new MailAddress(from);
                mailMessage.Subject = subject;
                mailMessage.Body = body;
            }
        }

        public static bool SendWelcomeMail()
        {
            string welcomeSubject = "Welcome as a new customer at Company!";
            string ourEmailAddress = "info@company.com";
            string bodyTemplate = "Hi {0}<br>We would like to welcome you as customer on our site!<br><br>Best Regards,<br>lcompany Team";

            try
            {
                foreach (var customer in DataLayer.ListCustomers())
                {
                    if (customer.CreatedDateTime >= DateTime.Now.AddDays(-1))
                    {
                        SendEmail(customer.Email, ourEmailAddress, welcomeSubject, string.Format(bodyTemplate, customer.Email));

#if DEBUG
                        // In debug mode, send email to
                        Console.WriteLine("Send mail to:" + customer.Email);
#else
    //Create a SmtpClient to our smtphost: yoursmtphost
                    System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("yoursmtphost");
                    Send mail
                    smtp.Send(mail);
#endif
                    }
                }
                return true;
                //All mails are sent! Success!
            }
            catch (Exception)
            {
                //Something went wrong :(
                return false;
            }
        }

        public static bool SendComeBackEmail(string voucherCode)
        {
            string comeBackSubject = "Welcome as a new customer at Company!";
            string ourEmailAddress = "info@company.com";
            string bodyTemplate = "Hi {0}" +
                                 "<br>We miss you as a customer. Our shop is filled with nice products. Here is a voucher that gives you 50 kr to shop for." +
                                 "<br>Voucher: " + voucherCode +
                                 "<br><br>Best Regards,<br>company Team";

            List<Customer> customers = DataLayer.ListCustomers();
            List<Order> customerOrders = DataLayer.ListOrders();

            try
            {
                foreach (var customer in DataLayer.ListCustomers())
                {
                    bool Send = false;
                    foreach (Order order in customerOrders)
                        if (customer.Email == order.CustomerEmail && order.OrderDatetime <= DateTime.Now.AddMonths(-1))
                        {
                            //We send email to that customer
                            Send = true;
                        }

                    //Send if customer hasn't put order
                    if (Send == true)
                    {
                        SendEmail(customer.Email, ourEmailAddress, comeBackSubject, string.Format(bodyTemplate, customer.Email));
#if DEBUG

                        Console.WriteLine("Send mail to:" + customer.Email);
#else
    //Create a SmtpClient to our smtphost: yoursmtphost
                    System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("yoursmtphost");
                    Send mail
                    smtp.Send(m);
#endif
                    }
                }
                //All mails are sent! Success!
                return true;
            }
            catch (Exception)
            {
                //Something went wrong :(
                return false;
            }
        }
    }
}

internal class Program
    {
        private static void Main(string[] args)
        {
            Console.WriteLine("Send Welcomemail");
            bool welcomeMailSuccess = SendingMail.SendWelcomeMail();
#if DEBUG
            //Debug mode, always send Comeback mail
            Console.WriteLine("Send Comebackmail");
            bool comeBackEmailSuccess = SendingMail.SendComeBackEmail("Thisisavouchercode");
#else
            //Every Sunday run Comeback mail
            if (DateTime.Now.DayOfWeek.Equals(DayOfWeek.Monday))
            {
                Console.WriteLine("Send Comebackmail");
                comebackEmailSuccess = SendComeBackEmail("Thisisavouchercode");
            }
#endif

            //Check if the sending went OK
            if (comeBackEmailSuccess == true)
            {
                Console.WriteLine("All mails are sent, I hope...");
            }
            //Check if the sending was not going well...
            if (comeBackEmailSuccess == false)
            {
                Console.WriteLine("Oops, something went wrong when sending mail (I think...)");
            }
            Console.ReadKey();
        }
    }

public class Customer
    {
        public string Email { get; set; }
        public DateTime CreatedDateTime { get; set; }
    }

    public class Order
    {
        public string CustomerEmail { get; set; }
        public DateTime OrderDatetime { get; set; }
    }

    class DataLayer
    {
        /// <summary>
        /// Mockup method for all customers
        /// </summary>
        public static List<Customer> ListCustomers()
        {
            return new List<Customer>()
                       {
                           new Customer(){Email = "mail1@mail.com", CreatedDateTime = DateTime.Now.AddHours(-7)},
                           new Customer(){Email = "mail2@mail.com", CreatedDateTime = DateTime.Now.AddDays(-1)},
                           new Customer(){Email = "mail3@mail.com", CreatedDateTime = DateTime.Now.AddMonths(-6)},
                           new Customer(){Email = "mail4@mail.com", CreatedDateTime = DateTime.Now.AddMonths(-1)},
                           new Customer(){Email = "mail5@mail.com", CreatedDateTime = DateTime.Now.AddMonths(-2)},
                           new Customer(){Email = "mail6@mail.com", CreatedDateTime = DateTime.Now.AddDays(-5)}
                       };
        }

        /// <summary>
        /// Mockup method for listing all orders
        /// </summary>
        public static List<Order> ListOrders()
        {
            return new List<Order>()
                       {
                           new Order(){CustomerEmail = "mail3@mail.com", OrderDatetime = DateTime.Now.AddMonths(-6)},
                           new Order(){CustomerEmail = "mail5@mail.com", OrderDatetime = DateTime.Now.AddMonths(-2)},
                           new Order(){CustomerEmail = "mail6@mail.com", OrderDatetime = DateTime.Now.AddDays(-2)}
                       };
        }