DIAGNOSTIC PLAYBOOK
Backup MX rejects recipients that the primary accepts
A backup MX must be configured to accept and route the domain's legitimate recipients. Merely publishing an alternate target does not establish its relay or recipient policy.
HealthCheck Email editorial team · · Examples are illustrative
How to investigate
- Verify which MX was contacted during the failed delivery and preserve its exact reply.
- Compare accepted domains and recipient validation between the primary and backup.
- Repair the backup's supported routing configuration and test a controlled failover path.
What this looks like
ILLUSTRATIVE EXAMPLE
The primary accepts invoices@example.com, but the backup has an older recipient list. Mail fails only when the primary is unavailable.
A mistake to avoid
Do not turn the backup into an unrestricted relay to avoid recipient rejections. Fix the intended domain and recipient configuration.
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.