Hello Yuenlok,
instead of the user "bulletin", I would use a different sender mailaddress and set up an alias for that address:
~alias/.qmail-bullbounce
Every mail that returns goes to bullbounce, you forward through a programm by putting "|/home/bulletin/bin/mail.pl" in ~alias/.qmail-bullbounce.
You can write a little Perl program to handle this:
#!/usr/bin/perl -w
use strict;
use Net::SMTP;
{
my $server = 'mailer.fredman.com';
my $sender = 'bullbounce@fredman.com';
my $receiver = 'bulletin@fredman.com';
my $smtp;
$smtp = Net::SMTP->new($server);
$smtp->mail($sender);
$smtp->to($receiver);
$smtp->data();
$smtp->datasend("From: $sender\n"

;
$smtp->datasend("To: $receiver\n"

;
$smtp->datasend("Subject: bounce-mail\n"

;
$smtp->datasend("\n"

;
$smtp->datasend(mailtext());
$smtp->dataend();
$smtp->quit();
}
sub mailtext {
my @mail;
while(<>) {
last if($_ eq 'Original message follows.');
push @mail, $_;
}
return @mail;
}
I hope this will do the job for you.