Virtual and redefining methods with "new" is not the same

virtual_and_new.cs

    public class ConcreteMockTest
    {
        [Test]
        public void Worker_WhenDoWorkIsCalled_SendsEmailToTylerAboutRussianBrides()
        {
            var emailSender = new FakeEmailSender();

            var worker = new Worker(emailSender);

            worker.DoWork();

            var message = emailSender.SentMessages.Last();

            Assert.AreEqual("Russian brides!", message.Body);
            Assert.AreEqual("tdu...@obsglobal.com", message.To[0].Address);
        }
    }

    class FakeEmailSender : EmailSender
    {
        public FakeEmailSender()
        {
            this.SentMessages = new List<MailMessage>();
        }

        new public void SendEmail(string toAddress, string body)
        {
            var m = new MailMessage("fr...@obsglobal.com", toAddress, "spam", body);
            SentMessages.Add(m);
        }

        public ICollection<MailMessage> SentMessages { get; private set; }
    }

    class Worker
    {

        private readonly EmailSender _notifier;

        public Worker(EmailSender notifier)
        {
            _notifier = notifier;
        }

        public void DoWork()
        {
            _notifier.SendEmail("tdu...@obsglobal.com", "Russian brides!");
        }
    }


    class EmailSender
    {
        public void SendEmail(string toAddress, string body)
        {
            var m = new MailMessage("tdu...@obsglobal.com", toAddress, "EMAILZ!", body);
            var client = new SmtpClient();
            client.Send(m);
        }

    }