How do fallback MX servers function?

Fallback (backup) MX servers provide redundancy:

Triggering failover: When the primary MX is unreachable or returns temporary errors, senders try higher priority (higher number) servers.

Queue and forward: Backup servers accept mail, queue it, and attempt delivery to the primary when it recovers.

Timeout handling: Senders typically wait several days before giving up, retrying through backup servers.

Security consideration: Backup servers must be properly secured. Attackers sometimes target backup MX as a weaker entry point.

Hosted email services often provide built-in redundancy. Self-hosted configurations need explicit backup server setup.

Secondary harbors accept cargo when the main port is closed, holding shipments until normal operations resume.