DIAGNOSTIC PLAYBOOK
Incoming email fails over the MX server's IPv6 address
An MX destination with an AAAA record advertises an IPv6 route that must be operational. A working IPv4 path does not establish that IPv6 SMTP routing, firewalls and service bindings are correct.
HealthCheck Email editorial team · · Examples are illustrative
How to investigate
- Resolve the destination's A and AAAA records and identify which family the failed connection used.
- Have the mail operator test port 25 reachability and SMTP service behavior on both addresses.
- Repair the IPv6 path or remove an unintended advertisement through the approved infrastructure process.
What this looks like
ILLUSTRATIVE EXAMPLE
The server publishes IPv6 DNS but listens for SMTP only on IPv4. Some sending systems encounter timeouts on the advertised IPv6 path.
A mistake to avoid
Do not assume every sender will fall back immediately or identically. Queue and retry behavior can make the impact intermittent.
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.