// Creating and initializing
var recipients = GetRecipientsList();

var notificationManager = new NotificationManager();
var emailServiceSettings = new EmailServiceSettings
{
    Host = "10.19.13.2",
    Port = 25,
    UserName = "user",
    Password = "password",
    EnableSsl = false,
};
var emailSender = new DefaultSmtpEmailSender();
emailSender.SetUpSettings(emailServiceSettings);
var emailNotifier = new EmailNotifier(emailSender);
notificationManager.Subscribers.Add(emailNotifier);
notificationManager.Notify(new NotifyObject());

// Declaration
 public class NotificationManager
    {
        public NotificationManager()
        {
            Subscribers = new List<IObserver<NotifyObject>>();
        }

        public List<IObserver<NotifyObject>> Subscribers { get; private set; }

        public void Notify(NotifyObject message)
        {
            Subscribers.ForEach(x => x.OnNext(message));
        }
    }

    public class EmailNotifier : IObserver<NotifyObject>
    {
        private IEmailSender _emailSender;

        public EmailNotifier(IEmailSender emailSender)
        {
            _emailSender = emailSender;
        }

        public void OnNext(NotifyObject notifyObject)
        {
            var email = new EmailContext
            {
                From = "file2rest@pixabit-solutions.de",
                EmailSubject = "File2Rest Notificator",
                Message = notifyObject.Message,
                Encoding = Encoding.UTF8,
                IsHtmlBody = false
            };

            _emailSender.SendEmail(new List<string> { "andrey.zherditskiy@pixabit-solutions.de" }, email);
        }

        public void OnError(Exception error)
        {
            throw error;
        }

        public void OnCompleted()
        {

        }
    }