Glossary
DNSBL — DNS-Based Blocklists
A DNSBL is a DNS-based blocklist: mail servers query a special DNS zone with a reversed IP embedded in the query. Example: for 1.2.3.4, query 4.3.2.1.zen.spamhaus.org — NXDOMAIN means not listed; an A record response means listed. Queries happen in real time during SMTP acceptance.
Delisting
Fix the cause (spam, compromised app, open relay) — then request delist per provider policy. Repeated listings hurt harder.
FAQ
Are DNSBLs the same as SPF?
No — DNSBL checks IP reputation; SPF checks authorised senders for a domain.
Why was my IP listed?
Spam complaints, spam trap hits, compromised hosts, or neighbors on bad ranges.
How do I check listings?
DNS Preflight Email tool and multi-DNSBL lookups.