var smtpClient = new SmtpClient();
var fromAddress = new MailAddress("from@email.ru"); 
var toAddress = new MailAddress("to@email.ru"); 
var mail = new MailMessage(fromAddress, toAddress)
        {
                IsBodyHtml = true,
                Subject = "TestTest"
        };
var htmlView = AlternateView.CreateAlternateViewFromString("html-код письма", Encoding.UTF8, "text/html");
mail.AlternateViews.Add(htmlView);
smtpClient.EnableSsl = false;
smtpClient.Send(mail);