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Enable DNSBL service in Postfix to reduce spam

!!! attention

If you're running a high-traffic mail server, you'd better setup a local
DNS server to cache DNS queries, because free RBL services like
`zen.spamhaus.org` may improperly reply if your server exceed the DNS
query limit. Also, mail service higly relies on DNS queries, so a local
DNS server speeds up the mail flow.

You can enable additional DNSBL services in Postfix to reduce spam. We use zen.spamhaus.org for example below.

  • Open Postfix config file /etc/postfix/main.cf or /usr/local/etc/postfix/main.cf (on FreeBSD), append reject_rbl_client zen.spamhaus.org to parameter smtpd_recipient_restrictions. Final setting looks like below:
smtpd_recipient_restrictions =
    ...
    reject_unauth_destination
    reject_rbl_client zen.spamhaus.org=127.0.0.[2..11]
    reject_rbl_client b.barracudacentral.org=127.0.0.2

It must be placed after reject_unauth_destination. You can add more DNSBL services after reject_unauth_destination, and they will be queried in the specified order.

Postfix will perform DNS query against zen.spamhaus.org, and wait for the response code, only 127.0.0.2 to 127.0.0.11 are meaningful, so we use =127.0.0.[2..11] to tell Postfix only reject clients when we get those response code.

  • If you have postscreen service enabled, you should add DNSBL services for postscreen service instead, so please don't use any reject_rbl_client in smtpd_recipient_restrictions parameter, but use below one instead:
postscreen_dnsbl_sites =
    zen.spamhaus.org=127.0.0.[2..11]*3
    b.barracudacentral.org=127.0.0.2*2
  • Restart or reload Postfix service is required.

See also

References