Postfix:
    maps_rbl_domains = work.drbl.caravan.ru, dul.ru, relays.ordb.org
    maps_rbl_reject_code = 550
    smtpd_client_restrictions = reject_maps_rbl
Sendmail (в sendmail.mc):
    FEATURE('dnsbl', 'dul.ru', 'Use mail relays of your ISP')dnl
    FEATURE('dnsbl', 'work.drbl.caravan.ru', `Use mail relays of your ISP')dnl
    FEATURE('dnsbl', 'relays.ordb.org', 'Spam bloked from open relay.')dnl
Ссылки: http://www.drbl.ofisp.org/zones.html , http://www.dul.ru/ ,  http://ordb.org/faq/