DIAGNOSTIC PLAYBOOK
Lowering DNS TTL during an email incident
Reducing a record's TTL does not retroactively shorten copies already cached with the old value. It helps future cache entries, so timing matters during a mail migration or rollback.
HealthCheck Email editorial team · · Examples are illustrative
How to investigate
- Record the old TTL, the change time and whether authoritative servers now agree.
- Estimate which earlier answers can still be cached and keep both service paths usable where possible.
- For future migrations, reduce TTL in advance and restore normal values after the transition stabilizes.
What this looks like
ILLUSTRATIVE EXAMPLE
A record had a long TTL when the outage began. Changing it to a short value now does not force every sender to discard yesterday's answer.
A mistake to avoid
TTL is not a universal delivery deadline. Sender queues, negative caching and policy caches can add separate timing behavior.
Keep the result in context
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.
Take the next step
Use the related check to gather evidence, then compare it with the affected message or service. Keep the result and time with your notes so a later change can be distinguished from the original problem.
Sources and further reading
The protocol references below explain the underlying behavior. Your sending or DNS provider supplies the account-specific settings for its service.