what is the best method to send div as email ?

Then do it like this:

using System.IO;
using System.Text;
using System.Net.Mail;

private string DivToHtml(HtmlGenericControl hgc)
{
StringBuilder sb = new StringBuilder();
StringWriter sw = new StringWriter(sb);
HtmlTextWriter hw = new HtmlTextWriter(sw);
hgc.RenderControl(hw);
return sb.ToString();
}