This simple php scrips can be run by cron job once an hour to let you know if the qmail queue is getting full.
$x = shell_exec('/usr/local/psa/admin/sbin/mailqueuemng -s');
$y = explode("\n", $x);
foreach($y as $z){
$a = explode(":", $z);
$out[trim($a[0])] = trim($a[1]);
}
if ($out['Messages total'] > 100) {
mail('[email protected]', 'Email Problems', print_r($out, true));
}