Ciao,
dando un'occhiata veloce devo farti un paio di osservazioni:
nella query vengono richiamati due campi: email e testo, a te non serve richiamare solo il campo email?
Non ho ben capito a cosa serve la variabile $body
Comunque, prova con questo codice:

Codice PHP:
$query="select distinct (email) from email";
//die("$cerca_email - email -> $email");
$result=mysql_query("$query");          

while($row=mysql_fetch_array($result))
{  
$email = $row["email"];   

$message .="\nInvio testo a -> $email";                 
$message .="\n\n\n\n Cordiali Saluti \n Assorisparxxxx";                 
$message .="\n-------------------------------------\n";                 
$message .="www.assorisparxxx.it\n";                                  
$message .="==================================\n";
                 
$email =strip_tags(trim($email));                 
$subject ="Estensione convenzione AssoRisparmio";                 
//$body =strip_tags(trim($body));                 
$from ="info@assorisparmio.it";                 
      
$headers ="Mime-Version: 1.0\r\nContent-Type:text/plain charset=\"iso-8859-1\"\r\n";                 
$headers .="Content-Transfer-Encoding: 7bit\r\n";                 
$headers .="X-Priority: 3\r\n";                 
$headers .="X-PHP-Mail-Priority: Normal\r\n";                 
$headers .="X-Mime-OLE: Produced by MS MimeOLE v5.00.3000.100\r\n";                 
$headers .="X-Mailer: PHP Mail generated by:PHPMailer v1.0\r\n";                 
$headers .="From: $from\r\n";                 
$headers .="Reply-to: $from\r\n";                 
                 
@$send=mail($email,$subject,$message,$headers);                                                            
}        

header("Location:[url]http://[/url]".$_SERVER['HTTP_HOST'].dirname($_SERVER['PHP_SELF'])."email.php?testo=ok");