MailMessage message = new MailMessage();
message.From = new MailAddress("from@server.com", "From Name");
message.To.Add(new MailAddress("to@server.com", "To Name"));
message.Subject = "Subject";

AlternateView textBody = AlternateView.CreateAlternateViewFromString("plain text body", null, "text/plain");
AlternateView htmlBody = AlternateView.CreateAlternateViewFromString("<b>HTML</b> body", null, "text/html");

message.AlternateViews.Add(textBody);
message.AlternateViews.Add(htmlBody);

SmtpClient emailClient = new SmtpClient("mail.server.com");
emailClient.Send(message);