DNS blocklists (DNSBL)

A DNS-based blacklist publishes whether an IP address or domain is listed using DNS queries. RFC 5782 describes how such lists are published and queried; it is not the same mechanism as SPF, DKIM, or DMARC.

Receivers that consult a DNSBL treat a positive listing as a signal about the connecting host or envelope domain. RFC 5782 discusses operational and architectural aspects of DNS blacklists and whitelists without mandating any particular list.

Not authentication

SPF, DKIM, and DMARC prove whether a message is authorized and aligned for the From domain. A DNSBL answers a different question: whether the connecting IP or a related name appears on a third-party reputation list the receiver chose to query.

RFC 5782 is informational. It documents how operators publish and consume DNSBL data; it does not replace the email authentication RFCs cited on the SPF and DMARC pages on this site.

Receiver choice

RFC 5782 does not mandate which DNSBL zones a receiver must query.

Listing on a DNSBL does not fix or replace failing SPF, DKIM, or DMARC checks at the message layer.

Delisting and list policy are defined by each list operator, not by the email authentication RFCs on this site.

Full version
RFC 5782 documents DNS-based blacklists and whitelists. Operators publish list membership in DNS; receivers query those zones during mail processing.
What is SPF
SPF is a TXT record that starts with v=spf1. It lists the hosts allowed to send mail for the domain. Two records that both start with v=spf1 on the same name are a permerror.
Reverse DNS (PTR)
A PTR record maps an IP address to a hostname in the in-addr.arpa or ip6.arpa tree. RFC 1035 defines PTR. Some receivers compare the PTR name to the EHLO hostname presented during SMTP.
What is DMARC
DMARC is a TXT record at _dmarc plus the domain. It tells the receiver what to do when the From address is not aligned with a passing SPF or DKIM result, and where to send the reports.