/// <summary>
/// Unsubscribes on sender.
/// </summary>
/// <param name="sender">Sender for unsubscribing.</param>
public void Unregister(TimeSender sender)
{
sender.NewMail -= Update;
}
/// <summary>
/// Subscribes on sender.
/// </summary>
/// <param name="sender">Sender for subscribing.</param>
public void Register(TimeSender sender)
{
sender.NewMail += Update;
}