php mail() doesnt know about bounced emails simply because it doesnt
reach it. there's no callback that allows you to register for bounced
emails. the email is received by your smtp server and is redirected
to the appropriate account (depending on the configuration)
you can get bounces and replies by adding some headers:
$mail_header .= "Return-Receipt-To: \"RETURNRECEIPT@yourdomain.org\" \n";
$mail_header .= "Return-Path: \"BOUNCE@yourdomain.org\" \n";
and then off course read out these addresses!