Enviar correo electrónico con formato HTML mediante C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net.Mail;

namespace EmailExample {
  public partial class Form1 : Form {
    public Form1() {
      InitializeComponent();
    }

    private void sendbtn_Click(object sender, EventArgs e) {
      try {
        MailMessage mail = new MailMessage();
        SmtpClient smtp = new SmtpClient("smtp.gmail.com");
        mail.From = new MailAddress(usertxt.Text);
        mail.To.Add(sendtotxt.Text);
        mail.Subject = subjecttxt.Text;
        var htmlbody = "Here your can write your html";
        mail.Body = htmlbody;
        smtp.Port = 587;
        smtp.Credentials = new System.Net.NetworkCredential(usertxt.Text, passtxt.Text);
        smtp.EnableSsl = true;
        smtp.Send(mail);
        MessageBox.Show("Mail Sent!");
      } catch (Exception ex) {
        MessageBox.Show(ex.Message);
      }
    }
  }
}