Notification service with email as an example.

EmailService.cs

namespace Demo.Notification
{
    using System.Net.Mail;

    public class EmailService : IEmailService
    {
        public void Notify(NotificationDto notification)
        {
            var msg = new MailMessage
                          {
                              Body = notification.Message,
                              From = new MailAddress(notification.ReplyTo),
                              IsBodyHtml = notification.IsHtml,
                              Subject = notification.Title
                          };

            foreach (string addressee in notification.Addressees)
            {
                msg.Bcc.Add(addressee);
            }

            using (var smtpClient = new SmtpClient { Host = "localhost" })
            {
                // Use Papercut [papercut.codeplex.com] for test server.
                smtpClient.Send(msg);
            }

            msg.Dispose();
        }
    }
}

NotificationDto.cs

namespace Demo.Business.DTOs
{
    public class NotificationDto
    {
        /// <summary>
        /// Gets or sets multiple recipients' addresses.
        /// </summary>
        /// <remarks>
        /// These could be email addresses or mobile phone numbers or 
        /// whatever fits with the <c>NotificationMedium</c>.
        /// </remarks>
        public string[] Addressees { get; set; }

        /// <summary>
        /// Gets or sets a value indicating whether the message is marked up with HTML.
        /// </summary>
        public bool IsHtml { get; set; }

        public string Message { get; set; }

        public string ReplyTo { get; set; }

        public string Title { get; set; }
    }
}

NotificationService.cs

namespace Demo.Notification
{
    public class NotificationService : INotificationService
    {
        public NotificationService(IEmailService emailService)
        {
            this.EmailService = emailService;
        }

        protected IEmailService EmailService { get; set; }

        public void NotifyIssueSaved(int issueId, string issueName, IEnumerable<PersonDto> users)
        {
            if (!this.Configuration.Enabled)
            {
                return;
            }

            var addresses = users.Select(user => user.Mail).ToArray();

            // create a notification.
            var notificationDto = new NotificationDto
                                      {
                                          Message = "Updated.",
                                          ReplyTo = "[email protected]",
                                          Addressees = addresses,
                                          Title = "Update"
                                      };

            // create a task (TPL) for each type of notification that the user's interested in.
            Task.Factory.StartNew(() => this.EmailService.Notify(notificationDto));
        }
    }
}