You would simply create a new email and send it off to the sender.
One way to do this would be to divert it to a script - for example using the transport table to catch the email, and the master table to pipe the output to a script to generate the bounce response.
The entry in the master table might look something like
bouncer
unix - n n - - pipe flags=Rq
user=nobody argv=/path/to/bounce.script ${sender} ${recipient}
Instead of sending sending the message to REJECT, you would send it to "bouncer". Not sure how you are rejecting it, so not sure how to tell it to go to bouncer instead. I guess you could use the transport table for this, or the header_checks table with the FILTER target.
A simple PHP script to send back the bounce message (untested) might look something along the following lines:
#! /usr/bin/php
<?php
# Make sure phpmailer is installed ! (apt-get install phpmailer
# or download from source and adjust path)
include phpmailer/class.phpmailer.php
$from=$argv[1];
$to=$argv[2];
$tz=date_default_timezone_get();
date_default_timezone_set($tz);
$message="Bounce Message text here";
$msg=new PHPMailer();
$msg->From=$to;
$msg->FromName=$from;
$msg->Subject="Bounce Notification Subject Line";
$msg->Body="Bounce Notification Message";
$msg->AddAddresses($from);
$msg->send();
?>