usa la funcion empty:
//create short variable names
$email=$HTTP_POST_VARS['url'];
$feedback=$HTTP_POST_VARS['descripcion'];
$toaddress = '
[email protected]';
$subject = 'Petición de añadir link';
$mailcontent = 'URL: '.$url."\n"
."Descripción: \n".$feedback."\n";
$fromaddress = 'From: Fomulario de anadir links';
if(empty($feedback)){
regresa al formulario
}else{
mail($toaddress, $subject, $mailcontent, $fromaddress);
}
saludos