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)
    {
        foreach (Customer customer in customers)
            Send(customer, body);
    }
    private void Send(Customer customer, string body)
    {
        MailMessage message = GenerateMessage(customer, body);
        int NumberOfRetriesOnError = 2;
        int DelayOnError = 10;
        for (int i = 0; i <= NumberOfRetriesOnError; ++i)
        {
            try
            {
                Sender.Send(GenerateMessage);
                return;
            }
            catch (SmtpException e)
            {
                if (i < NumberOfRetriesOnError)
                    Thread.Sleep((i + 1) * DelayOnError);
                else
                    Errors.Add(e.Message + customer.Email); // Include customeremail
            }
        }
    }
    private MailMessage GenerateMessage(Customer customer, string body)
    {
        string subject = "...";
        string from = "...";
        string to = customer.Email;
        MailMessage retVal = new MailMessage(from, to, subject, body);
        return retVal;
    }
}