$ cat /usr/local/vpopmail/etc/tcp.smtp
127.:allow,RELAYCLIENT=""
:allow,QMAILQUEUE="/var/qmail/bin/qmail-localfilter.sh",GREYLIST="Y"

$ cat /var/qmail/bin/qmail-localfilter.sh
#!/bin/sh
exec /usr/local/bin/qmail-qfilter ${SPAMC_BIN}/spamc ${SPAMC_FLAGS} -- ${QMAIL_LOCALFILTER_BIN}/qmail-localfilter.pl ${QMAIL_LOCALFILTER_FLAGS}

$ file /usr/local/bin/qmail-qfilter
/usr/local/bin/qmail-qfilter: ELF 64-bit LSB executable, x86-64, version 1 (OS), dynamically linked, interpreter /libexec/ld-elf.so.1, for OS x.y stripped

$ file /var/qmail/bin/qmail-localfilter.pl
/var/qmail/bin/qmail-localfilter.pl: a /usr/local/bin/perl script, ASCII text executable

qmail-qfilter, qmail-queue multi-filter front end
Installing qmail-qfilter

# [...]

$log_facility = "mail";
if($opt_s)
{
    $log_facility = $opt_s;
}

# [...]

# Begin by processing the mail header...
while(<>) {

  # [...]

  # Check for an "X-Spam-Flag: YES" line
  # If we find one, this piece of crap is busted.
  if (/^X-Spam-Flag: YES/oi) {
    $spam = 1;
    $opt_d and warn "DEBUG: Mail is spam!\n";
  }

  # [...]
}

#
# If this is spam, log that we have rejected it, (with the
# "from" and "to" lines if possible), include the reason why it
# was rejected, (if we have it), and exit with status code 31.
#
if ($spam) {
  setlogsock('unix');
  openlog('qmail-localfilter.pl','0',$log_facility);
  if ($have_from) {
    syslog('info',"Rejected spam $from $to");
  } else {
    if ($have_to) {
      syslog('info',"Rejected spam $to");
    } else {
    syslog('info',"Rejected spam.");
    }
  }
  if ($have_reason) {
    syslog('info',"$reason");
  }
  exit 31
}

# [...]