Ehm, Federix!
codice:
<script language="JavaScript" type="text/javascript">
function selectAll(value) {
var coll = document.getElementsByTagName("INPUT");
 if (coll!=null) {
    for (i=0; i<coll.length; i++) {
		 if (coll[i].type == 'checkbox' && coll[i].name != 'allbox') {
      coll[i].checked = value;
		 }
		}
 }
}

</script>

<form method="post" action="invia_mailing.asp" name="cutlist">
<input type="checkbox" name="allbox" value="all" onclick="selectAll(true);" />
<input type="checkbox" name="sendmail" value="1" />
<input type="checkbox" name="sendmail" value="2" />
<input type="checkbox" name="sendmail" value="3" />
<input type="checkbox" name="sendmail" value="4" />
<input type="checkbox" name="sendmail" value="5" />
<input type="submit" value="Via!" />
</form>
Scusa se mi sono intromesso! (in rosso l'errore)
Zappa