13 FOCUSED PLAYBOOKS
DNS & mail routing
Follow authoritative answers, cached records and MX destinations to find where a mail setup diverges.
The authoritative DNS service publishes records; recursive resolvers may retain earlier answers until their cache expires. Save the queried name, record type, resolver, answer and time. A website loading correctly does not establish that email records are present or that an SMTP server accepts mail.
Run an MX lookup →MX hostname has no A or AAAA record
An MX record needs a destination hostname that resolves to a usable address.
Read the playbook →DIAGNOSTIC PLAYBOOKMX points to a CNAME: correct the destination
An MX target should be a hostname with address records rather than a CNAME alias.
Read the playbook →DIAGNOSTIC PLAYBOOKEqual-priority MX servers receive different messages
MX servers with equal preferences may both receive traffic.
Read the playbook →DIAGNOSTIC PLAYBOOKBackup MX rejects recipients that the primary accepts
A backup MX must be configured to accept and route the domain's legitimate recipients.
Read the playbook →DIAGNOSTIC PLAYBOOKA new email record is still missing: check negative caching
Resolvers can cache an earlier missing-record answer.
Read the playbook →DIAGNOSTIC PLAYBOOKEmail DNS returns SERVFAIL: isolate the resolver failure
SERVFAIL indicates that a resolver could not complete the query successfully.
Read the playbook →DIAGNOSTIC PLAYBOOKAuthoritative nameservers disagree on email records
Different authoritative answers can make email checks appear inconsistent without any cache issue.
Read the playbook →DIAGNOSTIC PLAYBOOKEmail records were edited at the wrong DNS provider
A registrar account and the authoritative DNS service are not always the same place.
Read the playbook →DIAGNOSTIC PLAYBOOKIncoming email fails over the MX server's IPv6 address
An MX destination with an AAAA record advertises an IPv6 route that must be operational.
Read the playbook →DIAGNOSTIC PLAYBOOKNull MX mixed with ordinary MX records
A null MX explicitly states that a domain accepts no mail and is intended as the domain's only MX record.
Read the playbook →DIAGNOSTIC PLAYBOOKNo MX record: why delivery may still be attempted
The absence of MX records does not always mean senders immediately stop delivery attempts.
Read the playbook →DIAGNOSTIC PLAYBOOKLowering DNS TTL during an email incident
Reducing a record's TTL does not retroactively shorten copies already cached with the old value.
Read the playbook →DIAGNOSTIC PLAYBOOKInternal DNS passes but public email checks fail
Split-horizon DNS can return different answers inside and outside an organization.
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