Sending mail from .aspx page

hello friends,
I am looking for sending any server control in mail ..
but I am getting the Error which is:

Control 'TextBox1' of type 'TextBox' must be placed inside a form tag with runat=server.

this is CODE BEHIND:

protected void Page_Load(object sender, EventArgs e)
{
   TextBox1.Text = "Finishing";
   StringBuilder SB = new StringBuilder();
   StringWriter SW = new StringWriter(SB);
   HtmlTextWriter HTW = new HtmlTextWriter(SW);
   TextBox1.RenderControl(HTW);
}

this is ASPX page.