You could just stick your mail() function inside a loop. For instance:
if ($m < 1) {
$m = 1;
}
for ($i = 1; $i <= $m; $m++){
mail();
}
You can either have the user set how many emails to send, or you can just do it inside your script.
I'm not sure if that's exactly what you meant by "one at a time" though. If not...I have no idea.
Hope this hopes you some.