When recipients at a major mail provider begin returning 451 4.7.0 Temporary server error, the correct first response is usually observation, not aggressive queue flushing or sweeping DNS changes. The SMTP 451 class explicitly signals a temporary delivery problem.
Read the actual failure
Compare a transport-level failure such as connect: Network is unreachable with a remote server reply such as 451 4.7.0 following the DATA command. The first points toward local routing or connectivity; the second confirms that an SMTP server responded. It does not, by itself, prove a global service outage.
postqueue -p
postconf -h maximal_queue_lifetime
postconf -h minimal_backoff_time
postconf -h maximal_backoff_time
journalctl -u postfix --since '2 hours ago' --no-pager | tail -100
What not to do
Do not repeatedly run postqueue -f every few seconds, delete deferred messages, relax your anti-spam policy or rotate sending addresses without evidence. Those actions can cause load or reputational damage while the remote provider is degraded. Postfix already implements retry and exponential backoff behavior within its configured limits.
Track recovery
Measure deferred counts, age of the oldest message and the distribution of failed recipient domains. Check authoritative provider health communications for the same window and corroborate with successful deliveries to other destinations. If the remote service stabilizes, messages that remain within Postfix’s queue lifetime can be retried automatically.
Customer communication
Report which recipient domains are affected, distinguish queued from bounced messages, and state the configured maximum retention rather than promising eventual delivery. Include queue IDs only in private diagnostic reports, not public incident articles.