Add to /etc/postfix/main.cf:
    smtpd_reject_unlisted_recipient = no
    transport_maps = regexp:/etc/postfix/wg_mailer_transport
    wgml_destination_recipient_limit = 1

Add to /etc/postfix/master.cf:
    wgml      unix  -       n       n       -       -       pipe
        flags=FR user=martin argv=/data/custom/webgui_newsletter/sbin/newsletter-transport.pl ${domain} ${user}


Create /etc/postfix/wg_mailer_transport:
    /^[a-zA-Z0-9]+-bounce@/         wgml:

And run:
    postmap /etc/postfix/wg_mailer_transport

Finally restart postfix.