internal class Program
        {
            private static void Main(string[] args)
            {
                Sender.Errors.Clear();
                Sender.SendComeBackEmail("Thisisavouchercode");
                if (Sender.Errors.Any())
                    Console.WriteLine("All mails are sent, I hope...");

namespace EmailSender
{
    sealed class SendingMail
    {
        public List<string> Errors { get; } = new List<string>();

        public IEnumerable<Customer> Customers { get; set; }

        public IEnumerable<Order> Orders { get; set; }

        public IMailSender Sender { get; set; }

        public void SendWelcomeEmails()
        {
            var template = Resources.WelcomeEmailTemplate;
            Send(GetNewCustomers(), Resources.WelcomeEmailTemplate);
        }

        public void SendComeBackEmail()
        {
            var template = Resources.WelcomeEmailTemplate;
            var emailContent = String.Format(template);
            Send(GetCustomersWithoutRecentOrders(), Resources.ComeBackEmailTemplate);
        }

        private IEnumerable<Customer> GetNewCustomers()
        {
            var yesterday = DateTime.Now.Date.AddDays(-1);
            return Customers.Where(x => x.CreatedDateTime >= yesterday);
        }
        private IEnumerable<Customer> GetCustomersWithoutRecentOrders()
        {
            var oneMonthAgo = DateTime.Now.Date.AddMonths(-1);

            return Customers.Where(c => {
                var latestOrder = Orders
                    .Where(o => o.CustomerEmail == c.Email)
                    .OrderByDescending(o => o.OrderDatetime)
                    .FirstOrDefault();

                return latestOrder != null
                    && latestOrder.OrderDatetime < oneMonthAgo;
            });
        }

        private void Send(IEnumerable<Customer> customers, string body)
        {
            string titleOfEmail = "Welcome as a new customer at Company!";
            string ourEmailAddress = "info@company.com";
            int NumberOfRetriesOnError = 2;
            int DelayOnError = 10;

            foreach (var customer in customers)
            {
                for (int i = 0; i <= NumberOfRetriesOnError; ++i)
                {
                    try
                    {
                        Sender.Send(ourEmailAddress, customer.Email, titleOfEmail);
                        return;
                    }
                    catch (SmtpException e)
                    {
                        if (i < NumberOfRetriesOnError)
                            Thread.Sleep((i + 1) * DelayOnError);
                        else
                            Errors.Add(e.Message + customer); // Include customeremail
                    }
                }
            }
        }
    }
}

namespace EmailSender
{
    public interface IMailSender
    {
        void Send(string from, string to, string body);
    }
    sealed class NullMailSender : IMailSender
    {
        void IMailSender.Send(string from, string to, string body)
        {

        }
    }

    sealed class SmtpMailSender : IMailSender
    {
        void IMailSender.Send(string from, string to, string body)
        {
            System.Net.Mail.MailMessage mail = new System.Net.Mail.MailMessage();
            mail.From = new System.Net.Mail.MailAddress(from);
            System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("yoursmtphost");
            mail.To.Add(to);
            mail.Body = body;
            smtp.Send(mail);

        }
    }
}

namespace EmailSender
{
    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)}
                       };
        }
    }
}