[Mimedefang] md_check_against_smtp_server question

[An.H.Nguyen]

I try to avoid bounce messages by using md_check_against_smtp_server 
function.
When I tested I got smtp failure code 5.x.x for all "unkown user" messages, 
which is a good sign, but bounce messages still got generated on my 
mimedefang_server. How do I discard these "unkown user" messages?

Here's the filter code I use:
sub filter_recipient
{
      my($recip, $sender, $ip, $host, $first, $helo, $rcpt_mailer, 
$rcpt_host, $rcpt_addr) = @_;

      my $relay = undef;
      $relay = "internal_server" if ($recip =~ m/\@domain\.com$/i);
      if ($relay)
      {
              return md_check_against_smtp_server($sender, $recip, 
"mimedefang_server", $relay);
      }

      return('CONTINUE', "OK");  # accept recipient if dont find relay
}

====================================
from Mimedefang slide:
md_check_against_smtp_server runs the first bit
of an SMTP conversation to see if a local user exists
on another mail server. If you're running MIMEDefang
on a gateway, you can reject messages at the gateway
instead of being responsible for generating a bounce
message