Email recovery is broader than restoring mailbox data.
A service can be unavailable because of DNS, identity, certificates, gateways, connectors, provider outages, networking or administrative mistakes even when mailbox contents are intact.
Define the service
Document recovery objectives for inbound delivery, outbound delivery, mailbox access, application relay, authentication and administrative control.
Store-and-forward helps, but has limits
Remote MTAs normally retry temporary delivery failures, buying recovery time. That is not a substitute for a plan: retry periods are finite and critical outbound mail may still be unavailable.