This is the way I successfully fight spam using Postfix and Real-time Block Lists.
I used to use Sendmail with RBLs but a couple of years ago switched all my servers to Postfix since I found the latter easier to configure.
Add the fillowng lines to your Postfix main.cf file (on Linux usually at /etc/postfix/main.cf):
smtpd_helo_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_non_fqdn_hostname,
reject_invalid_hostname,
permit
smtpd_sender_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_non_fqdn_sender,
permit
smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_invalid_hostname,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_recipient_domain,
reject_unauth_pipelining,
reject_unauth_destination,
reject_rbl_client zen.spamhaus.org,
# reject_rbl_client no-more-funn.moensted.dk,
# reject_rbl_client virbl.dnsbl.bit.nl,
# reject_rbl_client bl.spamcop.net,
# reject_rbl_client dynamic.spamlists.tqmcube.com,
# reject_rbl_client list.dsbl.org,
# reject_rbl_client dnsbl-1.uceprotect.net,
# reject_rbl_client dnsbl-2.uceprotect.net,
# reject_rbl_client dnsbl-3.uceprotect.net,
# reject_rbl_client relays.ordb.org,
# reject_rbl_client lists.spam.sux.com,
# reject_rbl_client impersonator.lists.spam.sux.com,
# reject_rbl_client dnsbl.njabl.org,
# reject_rbl_client dnsbl.ahbl.org,
permit
You can see some commented out servers in my configuration file.
I’ve found them a bit restrictive for my purposes but you may find them useful.
You can find a good list of RBL servers on Multi-RBL lookup sites such as
CompleteWhois Multi-RBL Lookup Tool.