as stereofrog has already mentioned .. just pipe all orphan email (specific to that domain) to ur PHP script and use something like this to extract the email content:
$fd = fopen("php://stdin", "r");
$email = "";
while (!feof($fd)) {
$email .= fread($fd, 1024);
}
fclose($fd);
then extract the "to" field and if it belongs to a user .. forward the email to him.If you have cPanel .. this is even easier. goto mail > default address > set default address and instead of putting an email address there put something like this
"|php -q /home/whatever/public_html/pipe.php"
.. ofcourse without the quotes