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DNS

Domain Name System — the lookup fabric email authentication depends on.

Reading time:: 5 min
Difficulty:: Intermediate
Estimated duration:: 5 min
Prerequisites:: None
Last updated:: 2026-07-15
Author:: empire

Definition

Domain Name System — the lookup fabric email authentication depends on.

DNS zonesAdministrative units that hold DNS records for a domain.
TXT recordsDNS record type used for SPF, DMARC, and many auth artifacts.
CNAME recordsDNS aliases often used for DKIM selectors and vendor CNAMEs.
SubdomainsDelegated hostnames that inherit or override authentication policy.
MX recordsMail exchanger records that direct SMTP delivery for a domain.
A recordsIPv4 address records used by hosts and often by mail server names.
AAAA recordsIPv6 address records for hosts including mail servers.
NS recordsName server records that delegate authority for a zone.
SOA recordsStart of Authority records describing zone authority and timing parameters.
TTLTime to live — how long resolvers may cache a DNS answer.
DNS propagationThe period while caches converge after authoritative DNS changes.
Authoritative name serverA server that answers authoritatively for names in its zones.
Recursive resolverA resolver that performs iterative lookups and typically caches answers.
Glue recordsAddress records supplied with delegation to avoid circular dependencies.
PTR recordsPointer records used for reverse DNS lookups of IP addresses.
AuthenticationIdentity signals for email: SPF, DKIM, DMARC, and related controls.

CNAME at zone apexA CNAME is placed at the zone apex where other record types are also required.
HELO / EHLO problemsThe SMTP HELO/EHLO hostname is missing, invalid, or inconsistent with reverse DNS.
Inconsistent authoritative answersAuthoritative name servers for the same zone return conflicting data.
Incorrect MX targetMX points to the wrong host name or unreachable mail server.
Missing glue recordsDelegation references in-bailiwick name servers without required glue.
MX record missingNo MX record is published for a domain that should receive mail.
NXDOMAIN for expected nameA name that should exist returns NXDOMAIN from authoritative servers.
Reverse DNS mismatchPTR (reverse DNS) for a sending IP is missing or does not match the forward hostname used in mail.
Stale DNS cache after changeResolvers still serve prior answers after an authoritative change because TTL has not expired.
Wrong TTL for cutoverTTL values are too high for a planned cutover, prolonging mixed old/new answers.

digCLI DNS lookup utility for authoritative and recursive inspection of mail-related records.
nslookupInteractive DNS lookup utility commonly used for quick MX and TXT checks.
Google Admin Toolbox DigBrowser-based dig-style DNS lookup from Google’s Admin Toolbox.
Google Admin Toolbox CheckMXGoogle Admin Toolbox Check MX for inbound mail DNS sanity checks.

Open step-by-step: Verify DNS recordsVerify zone authority, MX/address records, and authentication-related TXT publications.

Empire Services

DNS for email readiness reviewReview MX, address records, delegation, and TTL strategy for mail domains.

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ReputationHow mailbox providers judge senders from IP, domain, and behavioral signals over time.

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