smtp      inet  n       -       n       -       -       smtpd
        -o content_filter=filter0

filter0    unix  -       n       n       -       10      pipe
   -o content_filter=filter1
   flags=Rq
   user=mail_filter
   argv=/usr/bin/filter0.sh -f ${sender} -- ${recipient}

filter1    unix  -       n       n       -       10      pipe
   flags=Rq
   user=mail_filter
   argv=/usr/bin/filter1.sh -f ${sender} -- ${recipient}