function ValidarEmail(email)
{
    if (/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(email)){
        return true;
    }
    else
    {
        return false;
    }
}
<!--\
function trim(string)
{
    string = string.toString();
    return string.replace(/(^\s*)|(\s*$)/g,'');
}
function send_form()
{
   //alert ("NELA");
   //alert(document.getElementById("Email").value);
	var mensaje = "The following error(s) occurred:\n";
	res = true;

    var checktext = document.getElementById("Checkcode").value.toUpperCase();
	if (trim(checktext) != "WORK SMART") {
            mensaje += "*Wrong Check Code.\n";
            res = false;
	}

	if (trim(document.getElementById("Email").value) == "")
        {
            mensaje += "*Email must contain an e-mail address.\n";
            res = false;
        }else if (!ValidarEmail(trim(document.getElementById("Email").value)))
        {
            mensaje += "*Email must contain an e-mail address.\n";
            res = false;
        }
	if (res == true) { 
		document.form1.submit();
	}else{ 
		alert(mensaje);
	}
}
-->