Email “451 temporary local problem” — find the server-side issue fast

What “451 temporary local problem” actually means

SMTP has two broad classes of failure: permanent (5xx) and temporary (4xx).
A 451 response is a 4xx: the sender is supposed to retry later. The phrase “temporary local problem”
is not a universal law; it’s a common textual explanation attached by MTAs (or intermediate policy/filters) when
the server can’t accept a message right now due to a local condition.

You’ll also see variants like:
451 4.3.0 Temporary server error,
451 4.3.2 System not accepting network messages,
or 451 4.7.1 Try again later.
The first number is SMTP status; the “enhanced status code” (like 4.3.0) gives a hint about category.
But here’s the uncomfortable truth: the textual part is often generic, and the enhanced code can be misleading
when a filter or policy daemon is the one speaking.

Where the error is generated matters

“451” might be coming from:

• The receiving MTA (Postfix, Exchange, Sendmail, Exim).
• A content filter (antivirus, antispam) that temporarily fails open or fails closed.
• A policy service (rate limiting, greylisting, reputation checks) that times out.
• An upstream relay (your smart host / outbound provider) returning 451 to you.
• Your own MTA while trying to deliver outbound mail, deferring to the queue.

Your first job is not to “fix 451.” Your first job is to answer one question:
Who said 451, at what stage, and because of which local resource?

One quote worth keeping on the wall: Hope is not a strategy. — paraphrased idea often attributed in engineering/ops circles.
451 is hope-based by default (“retry later”). We’re going to turn it into evidence.

Fast diagnosis playbook (first/second/third)

You want to find the bottleneck quickly, not write a love letter to every log file on the system.
Treat this like an incident: confirm scope, identify the failing component, then validate the most likely
constraints in order.

First: confirm where the 451 originates (10 minutes)

1. Is it inbound or outbound? Look at the message trace on the sending side or your own MTA logs.
   If you’re the sender, you’ll see “deferred” deliveries. If you’re the receiver, you’ll see incoming sessions rejected/deferred.
2. Capture the exact SMTP transcript snippet. You need the full line, including enhanced status and any bracketed reason text.
   “451 temporary local problem” without context is like “service degraded” without a graph.
3. Identify the component talking. Postfix might log “reject: 451” but the reason could be “policy service unavailable.”
   Exchange might map internal transport errors into 451 4.3.0.

Second: check the classic resource constraints (15 minutes)

1. Disk full / inode exhaustion: mail spools, queue directories, log partitions.
2. Queue growth: deferred queue exploding due to downstream issues.
3. DNS failures: resolver timeouts, broken caching, bad DNSSEC, or blocked UDP/53.
4. CPU / memory pressure: content filters timing out, policy daemons wedged, thread pools saturated.
5. Network/TLS: handshake timeouts, outbound port blocks, cert issues, MTU weirdness.

Third: check policy and dependencies (30–60 minutes)

1. Greylisting or rate limiting: intentional 451s masquerading as “local problem.”
2. Spam/AV integration: clamd, rspamd, Amavis, milter timeouts.
3. Database / directory dependencies: LDAP, SQL, Redis, reputation services.
4. Backpressure from upstream relay: your smarthost is deferring you (and you’re blaming yourself).

If you do only one thing: check disk and DNS first. Most 451 incidents are either
“we can’t write the message anywhere” or “we can’t resolve something we need right now.”

Interesting facts and historical context

• SMTP predates most modern reliability patterns. It was designed in an era where “retry later” was normal because networks were unreliable.
• 4xx is a feature, not a bug. Temporary failures are part of SMTP’s resilience model; senders should queue and retry with backoff.
• Enhanced status codes (like 4.3.0) came later. They were added to reduce ambiguity, but not all systems implement them consistently.
• Greylisting popularized intentional 451s. It deliberately returns temporary failures to see if the sender retries like a real MTA.
• Queue directories are performance-sensitive. MTAs historically used file-based queues; disk latency can become “temporary local problem” quickly.
• Content filtering moved the bottleneck. As spam rose, MTAs started delegating decisions to milters and scanners—adding new failure modes.
• DNS is in the critical path more than people think. Even when delivering by IP, many setups do reverse lookups, SPF checks, or policy queries.
• 451 is often used for reputation throttling. Some providers use 451 to slow or “soft block” senders without committing to a permanent rejection.

Joke #1: Email is the only protocol where “try again later” is a first-class success path. It’s like a restaurant that serves “maybe” with fries.

Practical tasks with commands (and what to decide)

These are server-side, production-safe checks. Each task includes: command, example output, what it means, and the decision you make.
Commands assume a Linux mail server (Postfix examples), but the logic carries over to other MTAs.

Task 1: Find the 451 in logs and extract context

cr0x@server:~$ sudo grep -R --line-number -E " 451 |451 " /var/log/mail.log | tail -n 20
/var/log/mail.log:128772:Jan  4 10:03:22 mx1 postfix/smtp[24188]: 9C2A8402C5: to=<user@example.net>, relay=mx.example.net[203.0.113.25]:25, delay=12, delays=0.1/0.02/3.1/8.8, dsn=4.3.0, status=deferred (host mx.example.net[203.0.113.25] said: 451 4.3.0 Temporary local problem (in reply to end of DATA command))

Meaning: This is outbound delivery. The remote host deferred after DATA (content phase), not at connect/HELO.
That points toward remote content filtering, queueing, or resource issues—less likely DNS.
Decision: If many recipients on the same domain show this, treat it as remote-side throttling and adjust retry/volume;
if it’s many domains, suspect your outbound reputation or your own content filter timing out.

Task 2: Check if it’s your inbound side rejecting clients

cr0x@server:~$ sudo grep -E "reject:.*451|NOQUEUE: reject:.*451" /var/log/mail.log | tail -n 20
Jan  4 10:05:09 mx1 postfix/smtpd[24501]: NOQUEUE: reject: RCPT from unknown[198.51.100.44]: 451 4.3.2 Temporary local problem; from=<sender@outside.tld> to=<user@yourdomain.com> proto=ESMTP helo=<mail.outside.tld>

Meaning: Inbound session is being deferred at RCPT time. That’s typically local policy, lookups, greylisting,
or a dependency outage (LDAP/SQL) that prevents recipient validation.
Decision: Stop looking at outbound queues; start checking your policy services and directory lookups.

Task 3: Check disk space where queues/spools live

cr0x@server:~$ df -hT
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/sda2      ext4   40G   39G  512M  99% /
/dev/sdb1      xfs   200G   45G  155G  23% /var

Meaning: Root filesystem is at 99%. If Postfix queue is under /var/spool/postfix and /var is fine, you might be okay—
unless logs or temp files are on /. Many setups still place critical paths on /.
Decision: If any filesystem hosting /var/spool, /var/lib, /var/log, or temp directories is >90%,
treat it as an active incident: free space, extend disk, or move spools. 451 can be your early warning before a hard 5xx.

Task 4: Check inode exhaustion (the silent disk-full)

cr0x@server:~$ df -ih
Filesystem    Inodes IUsed IFree IUse% Mounted on
/dev/sda2       2.6M  2.6M     0  100% /
/dev/sdb1        50M  1.2M  48.8M   3% /var

Meaning: Root has zero inodes. You can have “space available” and still be dead because the filesystem can’t create new files.
MTAs create lots of small files (queue entries). Inode exhaustion often triggers “temporary local problem” because file creation fails.
Decision: Find and delete inode-heavy junk (old temp files, crash dumps, runaway logs) or rebuild filesystem with more inodes
(ext4 mkfs options) if it’s chronic.

Task 5: Inspect Postfix queue size and deferred reasons

cr0x@server:~$ sudo postqueue -p | head -n 40
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
9C2A8402C5     3287 Thu Jan  4 10:03:21  no-reply@yourdomain.com
                                         user@example.net
                                         (host mx.example.net[203.0.113.25] said: 451 4.3.0 Temporary local problem (in reply to end of DATA command))

A1B7B40310     9121 Thu Jan  4 10:03:49  app@yourdomain.com
                                         ops@another.tld
                                         (connect to mx.another.tld[192.0.2.10]:25: Connection timed out)

-- 1245 Kbytes in 342 Requests.

Meaning: You have multiple failure modes: remote 451 and connection timeouts. This suggests broad outbound issues (network, reputation,
upstream provider) rather than a single remote domain problem.
Decision: If deferred count is rising quickly, throttle senders, pause bulk campaigns, and protect the queue volume from filling disks.
Then isolate: do you have DNS/connectivity issues or are you being temporarily blocked?

Task 6: Check the mail service health and recent restarts

cr0x@server:~$ systemctl status postfix --no-pager
● postfix.service - Postfix Mail Transport Agent
     Loaded: loaded (/lib/systemd/system/postfix.service; enabled)
     Active: active (running) since Thu 2026-01-04 09:58:12 UTC; 12min ago
   Main PID: 1218 (master)
      Tasks: 3 (limit: 18948)
     Memory: 22.1M
        CPU: 3.912s

Meaning: Postfix is running. That doesn’t mean it’s healthy, but it rules out the simplest “service down” scenario.
Decision: If there are repeated restarts, check for OOM kills, config reload loops, or dependency failures causing watchdog restarts.

Task 7: Look for OOM kills or kernel-level IO pain

cr0x@server:~$ sudo dmesg -T | egrep -i "oom|killed process|ext4|xfs|i/o error|blk_update_request" | tail -n 30
[Thu Jan  4 10:01:12 2026] Out of memory: Killed process 22219 (clamd) total-vm:2150340kB, anon-rss:824000kB, file-rss:0kB, shmem-rss:0kB

Meaning: Your antivirus daemon got killed. If your SMTP pipeline depends on clamd, the milter may time out or fail closed,
resulting in 451 deferrals.
Decision: Stabilize memory (limits, swap, container constraints), and decide whether mail should fail open (accept then scan async)
or fail closed (defer). In many regulated environments, fail closed is correct—but you must size it properly.

Task 8: Validate DNS resolution from the mail server (A/MX/PTR)

cr0x@server:~$ dig +time=2 +tries=1 mx example.net
;; ANSWER SECTION:
example.net.            300     IN      MX      10 mx.example.net.

cr0x@server:~$ dig +time=2 +tries=1 a mx.example.net
;; ANSWER SECTION:
mx.example.net.         300     IN      A       203.0.113.25

Meaning: DNS works quickly for this domain. If dig is slow or times out, your MTA will stall, queues will grow,
and some MTAs respond 451 when lookups for policy checks fail.
Decision: If DNS is slow, fix resolver health first (local caching, upstream reachability, firewall rules).
Don’t “tune Postfix” to compensate for broken DNS; that’s decorating a burning house.

Task 9: Check reverse DNS of your outbound IP (common trigger for throttling)

cr0x@server:~$ dig +short -x 198.51.100.10
mail01.yourdomain.com.

Meaning: PTR exists. Many providers won’t hard-fail on missing PTR, but they’ll throttle with 4xx responses.
Decision: If PTR is missing or wrong, fix it with your ISP/cloud provider. Expect this to reduce “mysterious 451s” at large receivers.

Task 10: Check policy daemon/milter responsiveness (socket connectivity)

cr0x@server:~$ sudo ss -lntp | egrep ":10023|:10024|:8891|:8892" || true
LISTEN 0      100          127.0.0.1:10024       0.0.0.0:*    users:(("master",pid=1218,fd=15))
LISTEN 0      100          127.0.0.1:8891        0.0.0.0:*    users:(("rspamd",pid=2110,fd=12))

Meaning: Your filtering ports are listening locally. That’s necessary, not sufficient.
Decision: If expected sockets aren’t listening, restart that subsystem and inspect its logs.
If sockets exist but time out, you likely have overload, lock contention, or downstream dependency latency.

Task 11: Measure local latency to the filter (a quick “is it slow?” test)

cr0x@server:~$ time (echo -e "QUIT\r\n" | nc -w 2 127.0.0.1 10024 >/dev/null)
real    0m0.006s
user    0m0.002s
sys     0m0.002s

Meaning: The port responds quickly. If this takes seconds or times out, your content filter path is delaying SMTP transactions,
and some clients/servers respond with 451 when their internal timeout budgets are exceeded.
Decision: If slow, scale the filter, tune timeouts, or temporarily bypass scanning for trusted sources while you stabilize.

Task 12: Check if you’re hitting connection limits or rate limits

cr0x@server:~$ sudo postconf | egrep "smtpd_client_connection_count_limit|smtpd_client_connection_rate_limit|smtpd_client_message_rate_limit"
smtpd_client_connection_count_limit = 50
smtpd_client_connection_rate_limit = 30
smtpd_client_message_rate_limit = 200

Meaning: Aggressive limits can trigger 4xx deferrals for legitimate bursts (password resets, incident alerts, batch jobs).
Decision: If you see bursts causing 451 for internal apps or trusted partners, raise limits and add per-client exceptions.
Keep limits for the internet at large; just don’t DDoS yourself with “security.”

Task 13: Inspect per-domain deferrals and concurrency (outbound shaping)

cr0x@server:~$ sudo postconf | egrep "default_destination_concurrency_limit|smtp_destination_concurrency_limit|smtp_destination_rate_delay"
default_destination_concurrency_limit = 20
smtp_destination_concurrency_limit = 20
smtp_destination_rate_delay = 0s

Meaning: High concurrency and no rate delay can look like abusive behavior to large providers, triggering temporary 451 throttling.
Decision: For domains known to throttle, add transport maps with lower concurrency and small rate delays. This is not “giving in”;
it’s being a polite neighbor so your mail arrives.

Task 14: Check the immediate health of the queue filesystem (latency)

cr0x@server:~$ sudo iostat -x 1 3
Device            r/s     w/s   rkB/s   wkB/s  await  svctm  %util
sda              2.00   85.00    64.0  4200.0  48.12   2.10  89.50
sdb              0.00   12.00     0.0   180.0   1.20   0.35   4.20

Meaning: sda is heavily utilized with high await. If your queue lives on sda, you have an IO bottleneck.
MTAs do lots of fsync-like behavior; queue IO latency turns into SMTP timeouts and 451 deferrals.
Decision: Move queue to faster storage, reduce sync pressure (carefully), or reduce queue churn (disable unnecessary logging,
avoid tiny filesystem partitions).

Task 15: Confirm TLS works and certificates aren’t causing stalls

cr0x@server:~$ openssl s_client -starttls smtp -connect mx1.yourdomain.com:25 -servername mx1.yourdomain.com -brief
CONNECTION ESTABLISHED
Protocol version: TLSv1.3
Ciphersuite: TLS_AES_256_GCM_SHA384
Peer certificate: CN = mx1.yourdomain.com
Verification: OK

Meaning: STARTTLS works and the certificate verifies locally (assuming your trust store is normal).
Decision: If this fails intermittently, check for expired certs, broken chain, or CPU contention during handshakes.
TLS issues can manifest as “temporary local problem” depending on where the failure is handled.

Task 16: Check if your upstream relay is the one deferring you

cr0x@server:~$ sudo grep -E "relay=.*\[.*\]:587|relay=.*:587" /var/log/mail.log | tail -n 10
Jan  4 10:04:11 mx1 postfix/smtp[24321]: 3F9B7402D2: to=<user@bigmail.tld>, relay=relay.provider.tld[192.0.2.50]:587, delay=4.2, dsn=4.7.1, status=deferred (host relay.provider.tld[192.0.2.50] said: 451 4.7.1 Rate limit exceeded, try again later)

Meaning: The “local problem” is not local. Your relay is throttling you.
Decision: Stop tuning your MTA blindly. Negotiate limits with the provider, implement outbound shaping, and verify you’re not
leaking spammy traffic from compromised accounts.

Root-cause map: where 451 comes from

“Temporary local problem” is a symptom bucket. Here’s the map I actually use in production: classify by stage and by dependency.
It reduces search space fast.

Stage-based classification

• At connect / banner: local listener overloaded, connection limits, accept queue full, kernel conntrack exhaustion, TLS policy issues.
• At HELO/EHLO: reverse DNS checks, helo restrictions, policy service queries, greylisting decision early.
• At MAIL FROM / RCPT TO: recipient lookup (LDAP/SQL), sender restrictions, SPF policy checks, rate limiting, greylisting.
• At DATA / end of DATA: content filter timeouts, antivirus scanning, spam scoring services, disk write/queue commit failures.
• After acceptance (bounce later): not 451; this is where you get DSNs or silent drops. Different problem.

Dependency-based classification (the usual suspects)

1) Storage and filesystem constraints

If Postfix can’t create queue files, can’t write logs, or can’t fsync in time, you’ll see temporary failures.
Storage issues show up as 451 because it’s safer to defer than to accept and lose mail.

Watch for:
disk full, inode exhaustion, high IO latency, saturated journal, NFS hiccups (yes, people still do this), or container overlays melting.

2) DNS and resolver instability

DNS is not optional. MTAs look up MX records, validate domains, sometimes do reverse DNS, and policy systems query TXT (SPF) and other data.
If your resolver times out, your SMTP transaction waits. If it waits too long, you defer. If you defer, you get 451.

3) Content filtering and milters

Antivirus and antispam are frequent “451 factories.” They tend to be CPU-heavy, memory-hungry, and dependency-rich (updates, rules, external reputation).
When they hiccup, they either:
fail closed (defer mail) or fail open (accept without scanning). Choose consciously, not by accident.

4) Policy services and directory lookups

Recipient verification, sender checks, rate limits, and authentication can call out to LDAP, SQL, Redis, HTTP APIs, or local daemons.
Any timeout becomes “temporary local problem.”

5) Remote throttling and reputation controls

Sometimes 451 is simply the remote side saying: “You’re sending too much” or “We don’t trust you yet.”
Large mailboxes do this to avoid mail floods and to force good behavior without permanent blocks.
Your best fix is usually outbound shaping plus reputation hygiene, not retrying harder.

Joke #2: The mail queue is like a gym membership—everyone loves having it, nobody loves checking how big it’s gotten.

Three corporate mini-stories (anonymized, plausible, useful)

Mini-story 1: The incident caused by a wrong assumption

A mid-sized SaaS company ran its own Postfix relay for transactional mail. They had a tight incident culture and a decent on-call rotation.
One Friday, support reported “email delays” from a handful of enterprise customers. The mail logs showed a lot of 451 deferrals.

The on-call engineer assumed it was the remote recipients throttling them. That assumption felt reasonable: it was end-of-DATA 451s,
and big enterprises do throttle. So they tweaked per-domain concurrency and went back to sleep.

Saturday morning, deferred queue had tripled. Not catastrophic, but trending in the wrong direction. The second engineer checked disk space
and found plenty. CPU looked fine. Network looked fine. Then they noticed something subtle: mail logs stopped updating every few minutes,
then burst-written in chunks.

Root cause: inode exhaustion on the root filesystem, caused by a quiet accumulation of tiny temporary files from an unrelated app.
Postfix itself lived on /var (healthy), but syslog was writing to /var/log on /, and when syslog couldn’t write, the team lost real-time visibility.
Meanwhile, the content filter was writing temp files to /tmp on /, failing intermittently, causing 451 at end-of-DATA.

Fix: clean up inode-heavy directories, move temp paths for the filter to /var, and add inode monitoring. The big lesson wasn’t “check inodes”—
it was: never accept a comforting external explanation (“remote throttling”) until you’ve proven your own system can write files reliably.

Mini-story 2: The optimization that backfired

A financial services org wanted faster mail throughput for internal notifications. Someone noticed the mail queue volume was on slower disks.
The proposed optimization: move /var/spool/postfix to a networked filesystem “for resilience” and free up local IO.
It passed a small test. It shipped.

Within a week, they saw intermittent 451 temporary local problem errors on inbound mail. Not constant—just enough to anger humans.
The errors clustered during backup windows and during sporadic network congestion. The queue would accept, then stall, then defer.

The postmortem was painful because nothing was down. The network filesystem was “up,” latency was “within SLA,” and backups were “successful.”
But SMTP is not impressed by your SLA; it cares about tail latency and fsync behavior. Queue operations that were sub-millisecond locally became
tens of milliseconds with occasional seconds-long stalls. That’s how you manufacture timeouts and 451s.

Fix: move the queue back to local SSD, keep resilience at the system level (replication, backups, rebuildable config),
and reserve network filesystems for things that don’t sit in the synchronous path of an SMTP transaction.
The “optimization” improved average numbers and destroyed p99. Email lives at p99.

Mini-story 3: The boring but correct practice that saved the day

A healthcare provider had strict security and compliance requirements, which meant heavy inbound scanning and conservative policy checks.
They also had a boring practice: every dependency of mail flow had a health check and an SLO, including DNS resolvers and the antispam service.
Nobody bragged about it at parties.

One Tuesday, they started seeing a spike in 451 deferrals at RCPT time. Users noticed quickly because appointment reminders were delayed.
The on-call engineer looked at the dashboard and saw the LDAP lookup latency graph had turned into a staircase.
At the same time, the SMTP service itself was fine.

They flipped a pre-approved “degraded mode” toggle: recipient verification switched from hard LDAP checks to cached lookups for known local domains
for 30 minutes, while keeping strict checks for external relaying. That reduced deferrals immediately.

Root cause: a routine directory maintenance job caused lock contention, turning fast lookups into timeouts. If they hadn’t had dependency SLOs,
the team would have chased Postfix tuning, firewall rules, and phantom spam attacks.

Fix: reschedule the maintenance job, add connection pooling and timeouts, and document the degraded mode with guardrails.
Boring won: a clean rollback path plus observability made 451 a 20-minute annoyance instead of a half-day incident.

Common mistakes: symptom → root cause → fix

These are the traps that keep teams stuck in the “it’s probably the other guy” loop. Be better than that.

1) Lots of 451 at end of DATA → scanner timeout → scale or bypass safely

• Symptom: 451 occurs “in reply to end of DATA command,” especially during busy periods.
• Root cause: antivirus/spam filter pipeline slow or unreachable; milter timeout; OOM kills.
• Fix: check filter processes, sockets, and timeouts; add capacity; adjust fail-open/closed policy explicitly; move temp directories to fast local disk.

2) 451 at RCPT time → recipient validation dependency broken → fix LDAP/SQL/DNS

• Symptom: “NOQUEUE: reject: RCPT … 451” and it correlates with directory latency.
• Root cause: recipient lookup service (LDAP/SQL) times out; policy daemon cannot reach its backend.
• Fix: restore dependency health; add caching; tune timeouts; ensure policy services degrade gracefully.

3) 451 bursts across many domains → DNS resolver issues → fix resolvers, not Postfix

• Symptom: deliveries defer for unrelated domains; logs show lookup delays or “temporary failure in name resolution.”
• Root cause: failing local resolver, blocked upstream DNS, overloaded caching daemon, bad DNSSEC behavior.
• Fix: validate resolver reachability; restart caching; use redundant resolvers; reduce DNSSEC breakage; monitor query latency.

4) Queue grows, but CPU seems fine → disk latency → relocate queue / fix IO

• Symptom: deferred queue grows; load average low; but delivery is sluggish.
• Root cause: storage tail latency; filesystem nearly full; inode exhaustion; slow journal commits.
• Fix: check iostat; move queue to SSD; free space; increase inode capacity; avoid network filesystem for queue.

5) Remote domains return 451 4.7.1 → throttling/reputation → shape traffic and clean up

• Symptom: remote or relay says “rate limit exceeded,” “try again later,” “temporarily deferred.”
• Root cause: too much concurrency, bursty campaigns, compromised sender, poor IP/domain reputation, missing PTR.
• Fix: implement per-domain rate limits; fix PTR and HELO; enforce outbound auth; rotate credentials; quarantine compromised accounts.

6) “We restarted Postfix and it went away” → hidden dependency resets → find the actual weak link

• Symptom: restart “fixes” 451 temporarily; returns later.
• Root cause: restarting resets queues, sockets, or DNS cache; underlying issue persists (memory leak, stuck filter, resolver overload).
• Fix: correlate with resource graphs; identify which subsystem recovers on restart; fix that component and add monitoring.

Checklists / step-by-step plan

When you get paged: 15-minute triage checklist

1. Get one concrete example: one sender, one recipient, one timestamp, one full SMTP error line.
2. Determine direction: inbound deferrals vs outbound deferrals.
3. Check disk + inodes on any filesystem involved in queue, logs, temp, and filters.
4. Check DNS speed from the MTA host, not your laptop.
5. Check queue growth: deferred count rising? active queue stuck?
6. Check filter/policy health: sockets listening, processes alive, recent OOM kills.
7. Check upstream relay responses: are you being throttled?
8. Choose containment: throttle senders, pause bulk mail, enable degraded mode for non-critical checks.

Containment actions (do these before “deep tuning”)

• Protect disk: if queue partition is at risk, stop non-essential mail sources (marketing, reports) before you fill it.
• Reduce concurrency: shape outbound to the affected domains or relay so you stop triggering throttles.
• Fail open vs fail closed: decide explicitly for filters. If you must fail closed, scale it now, not later.
• Extend retry intervals carefully: don’t create a retry storm. Backoff is a tool, not a hiding place.

Stabilization plan (same day)

1. Fix the real dependency: disk, DNS, filter, directory, or network.
2. Drain the queue safely: verify throughput and watch IO and latency while it drains.
3. Confirm with external delivery tests: pick a few representative recipient domains and validate end-to-end.
4. Put guardrails in place: monitoring for inodes, resolver latency, filter response time, and queue depth.

Hardening plan (this week)

• Separate filesystems: queue, logs, and temp should not fight each other on the same tiny partition.
• Set sane timeouts: policy daemons should fail fast, not hang SMTP sessions indefinitely.
• Introduce per-domain shaping: especially for known-throttling receivers.
• Audit who can send: outbound auth and rate limiting per account/app prevents one compromise from burning reputation.
• Run a quarterly “mail fire drill”: simulate DNS failure, disk near-full, and filter outage; confirm behavior matches your intent.

FAQ

1) Is 451 always the receiving server’s fault?

No. If you’re sending, the receiving server might be deferring you. But your own relay, policy services, scanners, DNS, or storage can also generate 451.
The log line tells you who said it—read the host ... said: portion carefully.

2) Why do some servers use 451 instead of a clear error?

Because SMTP expects retries, and deferring is safer than rejecting when the problem might clear (load spikes, temporary backend outage).
Also, some operators prefer soft blocks to manage abuse without creating permanent bounces.

3) What’s the difference between 451 and 421?

Both are temporary. 421 usually means “service not available” and often results in the server closing the connection.
451 is more “local error in processing” and can happen at specific stages (RCPT/DATA).

4) We only see 451 for one recipient domain. What should we do?

Assume throttling or a remote issue first, but validate basics: your outbound IP reputation, PTR, HELO name, and whether you’re sending bursts.
Implement per-domain concurrency limits and rate delay. If it persists, contact the recipient’s mail admins with timestamps and queue IDs.

5) Could greylisting be the cause?

Yes. Greylisting intentionally returns temporary errors (often 451 or 4.7.1 variants) to unknown senders.
If you run greylisting, confirm it’s not misconfigured or overloaded. If you’re the sender, confirm your system retries correctly and isn’t rotating source IPs.

6) Why does disk latency cause 451 instead of a more obvious “disk full” error?

Many MTAs and filters treat slow or failed writes as a transient condition to avoid losing mail.
If they can’t commit queue files quickly enough, they defer rather than accept and risk corruption or partial processing.

7) How long will senders retry after a 451?

It varies. Many MTAs retry for hours to days with exponential backoff. If your system is the one deferring, you’re pushing pain downstream.
Fix the local issue quickly; don’t rely on retries to “smooth it out” unless you’ve validated queue capacity and business impact.

8) Can SPF/DKIM/DMARC issues trigger 451?

Usually they cause permanent rejections (5xx) or acceptance with spam scoring. But in real systems, policy services that perform these checks can time out.
When the policy check times out, a server may return 451 even though the underlying “reason” is a slow DNS lookup for TXT records.

9) We restarted the filter and 451 stopped. Are we done?

You’re done for now. Find out why it needed restarting: memory leak, OOM kills, rule updates, disk IO, or dependency timeouts.
Add monitoring for the filter’s process health and response latency so the next failure isn’t a surprise cameo.

10) Should we increase Postfix timeouts to avoid 451?

Only if you’re sure the dependency is slow but correct and you have capacity to hold connections.
Increasing timeouts can convert “temporary failure” into “stuck sessions,” which is worse under load. Fix the dependency first.

Conclusion: next steps that prevent repeat incidents

“451 temporary local problem” is not a diagnosis; it’s an invitation to be systematic.
The fastest wins come from treating mail like any other production pipeline: identify the speaking component,
then check the constraints that cause deferral—storage, DNS, filters, policy backends, and upstream throttles.

Practical next steps:

1. Add inode monitoring on queue/log/temp filesystems, not just disk percentage.
2. Measure resolver latency from the MTA host and alert on timeouts.
3. Track queue depth and age: deferred count and oldest message age matter more than “service up.”
4. Instrument filters and policy daemons: response time, error rate, OOM kills.
5. Implement outbound shaping per major receiver domain and per upstream relay.
6. Write down your fail-open/closed choices for scanning and policy checks, then test them during a drill.

The goal isn’t to eliminate 451 forever. The goal is to make it boring: quick to attribute, quick to contain, and rare to repeat.