Simply set MailBuilder.Html property with your html, no need to do anything else:
MailBuilder builder = new MailBuilder();
builder.From.Add(new MailBox("alice@mail.com", "Alice"));
builder.To.Add(new MailBox("bob@mail.com", "Bob"));
builder.Subject = "This is HTML test";
builder.Html =
"This is an <strong>HTML</strong> message, " +
"<img src = 'http://www.example.com/image.jpeg' />.";
IMail email = builder.Create();
// Send the message
using (Smtp smtp = new Smtp())
{
smtp.Connect("server.example.com"); // or ConnectSSL for SSL
smtp.UseBestLogin("user", "password");
smtp.SendMessage(email);
smtp.Close();
}