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

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)}
                           };
            }
        }
    }

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);

                }
            }

public 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
                            }
                        }
                    }
                }
            }
        }