email and user unknow return of it

Hello

I'm new with PHP.
I tried a routine to send emails.
It was:

<?php

$Msg = "This is a test of the PHP email routinenn";
$Msg .= "If the destination addr is unknow this email must returnnn";
$Msg .= "to the addr at Reply-To parameter nn";

$Subject = "PHP Email Test";
$AddrTo = "unknowuser@xptokkyytt.com";

$Others = "Return-Path: <adm@superencontros.com.br>n";
$Others .= "Reply-To: <adm@superencontros.com.br>n";
$Others .= "From: "PHP Email Test" <adm@superencontros.com.br>n";

mail($AddrTo,$Subject,$Msg,$Others);

?>

The email goes by it don't return to the addr adm@superencontros.com.br if the destination
like (unknowuser@xptokkyytt.com) is an unknow user.

It returns to the ISP but not for me.

The web server is an Apache/1.3.9 Server running at a Linux system and the PHP is Version 3.0.12

I can talk to the webmaster and change any kind of parameter to solve this problem.

Thank for any solution.

ruitorres