Errore quando si esporta file excel

[Joisberg]

Salve,
Ho un problema quando vado ad esportare un lista di email massiccia (pi� di 3000 email).

<?php 
    include("config.php");    
    sql = mysql_query("SELECT nominativo,mail FROM clienti WHERE mail != '' ORDER BY mail ASC");

    if(mysql_num_rows($sql) > 0){ 
             $output1 = '            
              <table border="1">                
               <tr>                    
                <th bgcolor="#bfbfbf">Nominativo</th>                    
                <th bgcolor="#bfbfbf">Email</th>                
              </tr>';                

             echo $output1;                
             while($row = mysql_fetch_array($sql)){            
             $output2 ='            
                         <tr>                
                         <td>'.$row["nominativo"].'</td> 
                         <td>'.$row["mail"].'</td>    
                         </tr>            
                          ';                        

            echo $output2;        
            }                
         
             $output3 = '</table>';                

            header("Content-Type: application/xls");        
            header("Content-Disposition: attachment; filename=lista-email.xls");                
            echo $output3;    
          }

?>

Mi stampa correttamente la lista delle email, ma non mi esporta il file in *.xls e mi visualizza due errori che sono:
Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\clienti\esporta_email.php:25) in C:\xampp\htdocs\clienti\esporta_email.php on line 30

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\clienti\esporta_email.php:25) in C:\xampp\htdocs\clienti\esporta_email.php on line 31

Come posso risolvere?
Grazie in anticipo