How to add this plugin into your config/plugins:

# add this plugin BEFORE "rcpt_ok"
#
mailinglist_simple /usr/local/exim/virtual-domains/

# this plugin needs to run after all other "rcpt" plugins
rcpt_ok

Example config/mailinglist_simpe file:

# format:
#
# <email of mailinglists>       <only members can send email flag>      <path to signature file>

test@test.org                   1       /usr/local/exim/signatures/test
mylist@test.com                         /usr/local/exim/signatures/mylist
openlist@test.net

Here comes the plugin:

=head1 NAME
 
mailinglist_simple Version 1.2 by Marc Sebastian Pelzer http://search.cpan.org/~mpelzer/
 
=head1 DESCRIPTION
 
This plugin adds very simple mailinglist functionality to alias-file based distribution lists.
It reads its config-file config/mailinglist_simple and checks if the RCPT is a defined mailinglist.
If true, it adds a "Reply-to" header to the email before it gets queued. It also adds a "Precedence: bulk"
and "X-Mailing-List" header to prevent mail-loops/bounces from auto-reply applications.
 
Its also possible to define a mailinglist as PUBLIC or PROTECTED - meaning if everyone can send an email to
the list or just members of the list. You can also have a different signature for each list that will be
added to the end of the mail.
 
This plugin needs to be called in the config file config/plugins and must be BEFORE the "rcpt_ok" plugin!
 
If you want PROTECTED maillinglist support, you must call the plugin with one parameter - the path and/or
filename to your alias directory or alias file where your distribution lists are defined on the format:
 
localpart:      email@email.com,email2@email.com,email3@email.com
 
Example config extries:
 
mailinglist_simple
 
mailinglist_simple /usr/local/exim/virtual-domains/
 
mailinglist_simple /etc/aliases
 
Maybe you need to change the sourcecode to fit it to your alias file schema. Should not be too hard
if you know Perl :)
 
=cut
 
use Qpsmtpd::DSN;
 
sub register {
 
  my ($self, $qp, @args) = @_;
 
  if (@args > 0) {
 
    # check if we got an directory or file
    #
    if (-d $args[0] && -r $args[0]) {
 
      $self->{_ALIAS_DIRECTORY} = $args[0];
      $self->{_PROTECTED_SUPPORT} = 'true';
 
      $self->log(LOGINFO, "OK. Using alias directory '". $args[0] . "'");
 
    } elsif (-f $args[0] && -r $args[0]) {
 
      $self->{_ALIAS_FILE} = $args[0];
      $self->{_PROTECTED_SUPPORT} = 'true';
 
      $self->log(LOGINFO, "OK. Using alias file '" . $args[0] . "'");
 
    } else {
 
      die "Bad argument - need a valid alias directory or full path to a file: $args[0]";
    }
 
  } else {
 
    $self->{_PROTECTED_SUPPORT} = 'false';  
  
  }
}
 
sub hook_rcpt {
 
    my ($self, $transaction, $recipient) = @_;
    my ($element, $list, $domain, $is_protected, $emails, $localpart, @emails, $line, $tmp, $isAValidSender, $signature);
 
    my @mailinglists = $self->qp->config("mailinglist_simple");
    unless (@mailinglists) { $self->log(LOGDEBUG, "No configuration file config/mailinglist_simple found."); return DECLINED; }
 
    my $host = lc $recipient->host;
    my $from = lc($recipient->user) . '@' . $host;
 
    foreach $element (@mailinglists) {
 
        chomp($element);
 
        if ($element =~ m!^\s*\#! || $element =~ m!^\n$!) { next; }     # ignore comments
 
        # split line into pieces
        #
        ($list, $domain, $is_protected, $signature) = ($element =~ m!^\s*([^\@]+)\@([\w\d\-\_\.]+)\s*([\d]?)\s*(\S*)!);
 
        # try to mach RCPT TO and defined lists
        #
        if ($list eq lc($recipient->user) && $domain eq $host) {
 
            if ($is_protected =~ m!^\d$! && $self->{_PROTECTED_SUPPORT} eq 'true') {
 
                # check if SENDER is member of the lists
                #
                $self->log(LOGNOTICE, "Found email to a PROTECTED mailinglist -> $from - checking SENDER");
 
                if ($self->{_ALIAS_DIRECTORY}) {
 
                    unless (-r $self->{_ALIAS_DIRECTORY} . "/$host") { $self->log(LOGWARN, "Error while try to open ALIAS file '" . $self->{_ALIAS_DIRECTORY} . "/$host"); return DENYSOFT; }
 
                    open (ALIAS, $self->{_ALIAS_DIRECTORY} . "/$host");
 
                } elsif ($self->{_ALIAS_FILE}) {
 
                    unless (-r $self->{_ALIAS_FILE}) { $self->log(LOGWARN, "Error while try to open ALIAS file '" . $self->{_ALIAS_FILE}); return DENYSOFT; }
                    
                    open (ALIAS, $self->{_ALIAS_FILE});
 
                }
 
                  while ($line = <ALIAS>) {
 
                    chomp($line);
 
                    if ($line =~ m!^\s*\#! || $line =~ m!^\n$! || ! $line) { next; }
 
                    ($localpart, $emails) = ($line =~ m!^\s*([^\:\s]+)\s*\:\s*(.+)$!);
 
                    if ($localpart eq $list) {
 
                        (@emails) = split(",", $emails);
 
                        foreach $tmp (@emails) { $tmp =~ s!\s!!g; if (lc($tmp) eq lc($transaction->sender->address())) { ++$isAValidSender; } }
                    }
                  }
 
                  close (ALIAS);
 
                  if ($isAValidSender >= 1) {
 
                    $self->log(LOGNOTICE, "Sender '" . $transaction->sender->address() . "' is a member of mailinglist '$from'. Accepting.");
                    
                    $transaction->notes("mailinglist", $from);      # putting email into transaction notes to write the "Reply-to" header later...
 
                    # add signature - if defined and available in the file-system
                    #
                    if (-e $signature && -r $signature) { $self->log(LOGNOTICE, "Adding signature '$signature'."); $transaction->notes("mailinglist_signature", $signature); }
 
                    return DECLINED;
                    
                  } else {
                  
                    $self->log(LOGWARN, "Sender '" . $transaction->sender->address() . "' is NOT a member of mailinglist '$from'. Rejecting.");
                    
                    return Qpsmtpd::DSN->no_such_user("mail to $from not accepted from you");
                  }
                  
            } else {
 
                # this is an PUBLIC list - everyone can send an email to it
                #
                $self->log(LOGNOTICE, "Found email to a PUBLIC mailinglist '$from'. Accepting.");
 
                $transaction->notes("mailinglist", $from);      # putting email into transaction notes to write the "Reply-to" header later...
 
                # add signature - if defined and available in the file-system
                #
                if (-e $signature && -r $signature) { $self->log(LOGNOTICE, "Adding signature '$signature'."); $transaction->notes("mailinglist_signature", $signature); }
 
                return DECLINED;
            }
        }
    }
 
    return DECLINED;
}
 
sub hook_data_post {
 
    my ($self, $transaction) = @_;
    my ($line, $signature);
 
    unless ($transaction->notes("mailinglist")) {
 
        $self->log(LOGDEBUG, "Email is not an email to a mailinglists. Ignoring.");
 
        return DECLINED;
    }
 
    $self->log(LOGNOTICE, "Email is an email to a mailinglist -> " . $transaction->notes("mailinglist") . ". Adding Reply-to header element.");
 
    my $headers = $transaction->header();
 
    if ($headers->get("Precedence") =~ m!(bulk|list|junk)!i || $headers->get("X-Mailing-List") || $headers->get("X-loop") eq $transaction->notes("mailinglist") || $headers->get("Auto-Submitted")) {
 
        $self->log(LOGWARN, "Loop detected or email from another mailinglist or auto-reply agent. Ignoring mail!");
 
        return (DENY, "Loop detected!");
    }
 
    $headers->replace("Reply-to", $transaction->notes("mailinglist"));
    $headers->replace("Sender", $transaction->notes("mailinglist"));
 
    # add header elements to prevent loops from auto-replies
    #
    $headers->add("Precedence", "bulk");
    $headers->add("X-Mailing-List", "qpsmtpd mailinglist");
    $headers->add("X-loop", $transaction->notes("mailinglist"));
 
    #$headers->replace("Envelope-From", "");        # we will add envelope handling later ...
 
    if ($transaction->notes("mailinglist_signature")) {
 
        open (SIG, $transaction->notes("mailinglist_signature"));
        while ($line = <SIG>) { $signature .= $line; }
        close (SIG);
 
        $transaction->body_write("\n\n--\n" . $signature);
    }
 
    return DECLINED;
} 
 
1;