onsubmit

 <script type="text/javascript">
  function disable_form_submit(form){
    inputs = form.getElementsByTagName('input');
    for (i = 0; i < inputs.length; i++){
      if (inputs[i][0]) {
        for (j = 0; j < inputs[i].length; j++){
          if (inputs[i][j].type == 'submit'){
            inputs[i][j].disabled = true;
          }
        }
      } else {
        if (inputs[i].type == 'submit'){
          inputs[i].disabled = true;
        }
      }
    }
  }
  </script>
..........
<form method="post" action ="newmail.php" onsubmit="disable_form_submit(this)">