Ok got the answer
in the file qa-include/qa-app-maling.php there is a line which reads
$lasttime=max($lasttime, $thistime-6); // ... don't do more than 6 seconds' worth
which means it doesn not wait more than 6 sec interval ( 10 emails per minute)
change 6 to a higher number, say 10!
$lasttime=max($lasttime, $thistime-10); // ... don't do more than 6 seconds' worth