Email: “554 spam detected” — clean up reputation without wasting weeks

What “554 spam detected” actually means (and what it doesn’t)

SMTP 554 is a permanent failure. In practice, it means the receiving server decided your message is not worth accepting—often due to spam filtering, authentication failures, policy checks, reputation, content patterns, or traffic anomalies. “Spam detected” is a human-friendly gloss; the underlying reason may be anything from a misaligned DKIM signature to a compromised host spewing to invalid addresses.

554 is not a root cause, it’s a verdict

Think of 554 like “HTTP 403 Forbidden.” It tells you the request was denied, not whether it was your credentials, your IP reputation, or your behavior that triggered the denial. Some providers stuff more detail in the enhanced status code (like 5.7.1) and the text string. Many don’t. Your job is to correlate their rejection with your evidence: logs, DNS, headers, rates, bounces, and user reports.

What it is not

• Not “the recipient’s mailbox is full” (that’s usually 552 or a 4xx deferral).
• Not a transient queue spike you can “just retry forever.” Retrying 5xx like it’s a 4xx is how you turn a deliverability problem into a self-inflicted DDoS of your own reputation.
• Not fixed by buying a new domain every time. You can, but then your customers see you as the kind of company that changes caller ID weekly.

And one joke, because we’re going to be here a while: Deliverability is like hydrating—if you wait until you’re thirsty, you already messed up.

Fast diagnosis playbook (first/second/third checks)

This is the “I have 30 minutes to stop the bleeding” sequence. Don’t freestyle. Don’t start with “rewrite the email template.” Start with evidence that separates identity, reputation, and behavior.

First: Identify what’s being rejected (scope)

• Is it one provider (Gmail only, Microsoft only), or all of them?
• Is it one stream (marketing bulk) or everything (transactional resets)?
• Is it one IP (new node) or the whole outbound pool?
• Is it one domain (new vanity domain) or all sending domains?

Decision: if it’s one provider, focus on that provider’s policy + reputation signals. If it’s everyone, assume your authentication or sending host is misbehaving.

Second: Check authentication alignment (SPF, DKIM, DMARC)

• Confirm the domain in the From: aligns with SPF or DKIM identity (DMARC). Misalignment is the fastest way to look like an impersonator.
• Confirm DNS is correct, public, and not stale across split-horizon setups.

Decision: if alignment fails, fix DNS and signing before you do anything else. Warming up a broken identity just teaches filters to distrust you faster.

Third: Check reputation & list hygiene signals

• Look for spikes in volume, spikes in bounces, and spammy recipient patterns (role accounts, harvested lists, random domains).
• Check for blocklist hits and whether your IP has a clean reverse DNS and stable HELO/EHLO.

Decision: if bounces are high or you hit a major blocklist, pause bulk traffic and isolate transactional mail onto the cleanest possible path.

Fourth: Only then look at content

Content can matter, especially for phishing-like templates, URL shorteners, and weird MIME. But content is usually not the first domino. Treat it as a multiplier, not the root.

Interesting facts & history: why mailbox providers behave like this

• Fact 1: SMTP predates modern abuse controls; early email assumed a friendly network. Anti-spam bolted on later as policy engines and reputation systems.
• Fact 2: The first widely-used DNS-based blocklists emerged in the late 1990s as a reaction to open relays—systems that would forward mail for anyone.
• Fact 3: SPF gained traction in the early 2000s as a way to say “these IPs may send for my domain,” but it never authenticated the message content—just the envelope path.
• Fact 4: DKIM’s big idea was domain-level cryptographic signing so providers could build reputation on a stable identity even when IPs change.
• Fact 5: DMARC (2012 era) forced the industry to care about alignment—not just “SPF passed somewhere,” but “SPF/DKIM passed for the same domain users see.”
• Fact 6: Many providers treat spam complaints as stronger signals than bounces. One angry “Report spam” click can outweigh dozens of deliveries.
• Fact 7: Gmail and Microsoft use massive behavioral models: sending patterns, engagement, complaint rate, and reputation across related domains and IP ranges.
• Fact 8: IPv6 deliverability is still uneven; some receivers are stricter on IPv6 because abuse is easier to scale when address space is effectively infinite.

The real failure modes behind 554

1) Authentication is technically “present” but operationally broken

The most painful incidents are the ones where SPF exists, DKIM exists, DMARC exists… and you still fail alignment because of one small mismatch: wrong selector, bad canonicalization, expired key rotation, forwarding path, or an innocent vendor sending as you without being authorized.

2) Reputation tanked from traffic behavior, not content

Providers hate surprises. A new IP that goes from 0 to 50k/day looks like a botnet. A stable IP that suddenly starts hitting lots of invalid recipients looks like list acquisition. A host that keeps retrying 5xx aggressively looks like you don’t respect policy or you’re broken.

3) Backscatter and blowback from misconfigured bounces

If your system generates non-delivery reports (NDRs) to forged senders, you’re sending spam inadvertently. Receivers notice. They are not amused.

4) Infrastructure identity mismatch (PTR, HELO, TLS posture)

Mail servers are judged like suspicious travelers: name, papers, and story must match. If your IP reverse DNS says one thing, your HELO says another, and your certificate looks like it was minted in a basement, you’ll get more scrutiny.

5) Compromise: one box, one credential, one “helpful” script

The classic: a webserver gets popped, attacker sends mail through localhost or stolen SMTP credentials, and your domain becomes the world’s least fun newsletter. You’ll see spikes, strange destinations, and angry rejections. Treat this as a security incident first.

Second joke (and last, per the rules): Nothing boosts your “spammy” aura like sending 30,000 password resets nobody asked for.

Hands-on tasks: commands, outputs, and decisions (12+)

Below are practical tasks you can run on a typical Linux MTA host (Postfix examples, but most concepts carry). Each task includes: command, what output means, and the decision you make.

Task 1: Confirm the exact rejection text and enhanced status code

cr0x@server:~$ sudo grep -E "status=bounced|status=deferred" /var/log/mail.log | tail -n 20
Jan 03 10:14:12 mx1 postfix/smtp[21844]: 3A1B012345: to=<user@gmail.com>, relay=gmail-smtp-in.l.google.com[142.250.102.26]:25, delay=1.2, delays=0.1/0.0/0.4/0.7, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[142.250.102.26] said: 554 5.7.1 Message rejected as spam (in reply to end of DATA command))

Meaning: This is a hard fail (5.7.1 policy/spam). The “end of DATA” tells you it accepted the SMTP envelope but rejected after content/policy evaluation.

Decision: Stop retries for these recipients; pivot to authentication/reputation checks and isolate streams.

Task 2: Count rejections by destination domain (find scope fast)

cr0x@server:~$ sudo awk -F'to=<' '/status=bounced|status=deferred/ {print $2}' /var/log/mail.log | awk -F'>' '{print $1}' | awk -F'@' '{print $2}' | sort | uniq -c | sort -nr | head
  412 gmail.com
  133 outlook.com
   58 yahoo.com
   21 icloud.com
    9 example-corp.com

Meaning: Concentration at big providers suggests reputation/auth issues. If it were mostly one corporate domain, you might be dealing with a specific gateway policy.

Decision: Prioritize Gmail/Microsoft remediation and validate global identity signals.

Task 3: Check outbound volume trend (is this a spike?)

cr0x@server:~$ sudo grep "postfix/qmgr" /var/log/mail.log | awk '{print $1,$2,$3}' | uniq -c | tail -n 5
   98 Jan 03 10:10:01
  101 Jan 03 10:11:01
  544 Jan 03 10:12:01
  610 Jan 03 10:13:01
  588 Jan 03 10:14:01

Meaning: A sudden jump from ~100/min to ~600/min is a reputation hazard, especially on a new IP.

Decision: Rate-limit now; if you must deliver critical mail, isolate it to a slower lane with clean lists.

Task 4: Verify the public IP you’re actually sending from

cr0x@server:~$ curl -s ifconfig.me
203.0.113.45

Meaning: Confirms NAT/egress IP. Many teams debug the wrong address because cloud egress differs from the instance IP.

Decision: Use this IP for reverse DNS, blocklist checks, and provider postmaster tooling.

Task 5: Validate reverse DNS (PTR) matches your sending identity

cr0x@server:~$ dig +short -x 203.0.113.45
mx1.mail.example.com.

Meaning: You have a PTR. Good. Now make sure forward DNS for that hostname points back to the same IP (next task).

Decision: If PTR is missing or generic, fix it with your provider. No PTR is not always fatal, but it’s a frequent “spammy” multiplier.

Task 6: Confirm forward-confirmed reverse DNS (FCrDNS)

cr0x@server:~$ dig +short mx1.mail.example.com A
203.0.113.45

Meaning: PTR and A record agree. This consistency reduces suspicion and avoids some receiver-side heuristics.

Decision: If it doesn’t match, align it. Avoid having PTR to one name and HELO to another.

Task 7: Check your HELO/EHLO name (it should be real and resolvable)

cr0x@server:~$ sudo postconf -n | grep -E '^myhostname|^smtp_helo_name'
myhostname = mx1.mail.example.com
smtp_helo_name = $myhostname

Meaning: Your HELO name is stable and matches DNS identity. If HELO is “localhost” or a random internal name, some receivers will penalize you.

Decision: Set myhostname to the PTR name and ensure it resolves.

Task 8: Confirm SPF record for the visible From domain

cr0x@server:~$ dig +short TXT example.com | sed 's/"//g'
v=spf1 ip4:203.0.113.45 include:_spf.mailvendor.example -all

Meaning: SPF explicitly allows your sending IP and your vendor. The -all is strict (good when correct).

Decision: If your IP isn’t authorized, add it (or fix routing so you send from authorized infrastructure). Don’t leave SPF as ~all forever out of fear—be correct instead.

Task 9: Check DKIM public key record exists (selector)

cr0x@server:~$ dig +short TXT s1._domainkey.example.com | head -n 2
"v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A..."

Meaning: DKIM selector s1 is published. If you rotated keys and forgot DNS, you’ll get silent DKIM fails.

Decision: If missing, publish the correct key and ensure your MTA signs with the same selector.

Task 10: Inspect a delivered message header for SPF/DKIM/DMARC results

cr0x@server:~$ grep -E "Authentication-Results|Received-SPF|DKIM-Signature" -n sample-headers.txt | head -n 20
12: Authentication-Results: mx.google.com;
13:        dkim=pass header.i=@example.com header.s=s1 header.b=abc123;
14:        spf=pass (google.com: domain of bounce@example.com designates 203.0.113.45 as permitted sender) smtp.mailfrom=bounce@example.com;
15:        dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=example.com
28: DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=s1; c=relaxed/relaxed; ...

Meaning: This is the gold standard: SPF pass, DKIM pass, DMARC pass. If you see dmarc=fail while SPF/DKIM pass, alignment is broken (common with third-party senders using their own bounce domain).

Decision: If DMARC fails, fix alignment: ensure the DKIM d= matches the From domain, or ensure SPF mailfrom aligns.

Task 11: Check DMARC policy and reporting addresses

cr0x@server:~$ dig +short TXT _dmarc.example.com | sed 's/"//g'
v=DMARC1; p=reject; rua=mailto:dmarc-rua@example.com; ruf=mailto:dmarc-ruf@example.com; adkim=s; aspf=s

Meaning: Strict alignment (adkim=s, aspf=s) is great if you’re consistent. It’s painful if you have legacy systems sending from subdomains or vendors not aligned.

Decision: If you’re failing due to strict alignment, either fix senders to align or relax to r temporarily while you migrate. Don’t leave it relaxed forever.

Task 12: Verify the TLS posture presented by your SMTP server

cr0x@server:~$ openssl s_client -starttls smtp -connect mx1.mail.example.com:25 -servername mx1.mail.example.com </dev/null 2>/dev/null | openssl x509 -noout -subject -issuer -dates
subject=CN = mx1.mail.example.com
issuer=CN = R3, O = Let's Encrypt, C = US
notBefore=Dec 10 00:00:00 2025 GMT
notAfter=Mar 10 23:59:59 2026 GMT

Meaning: A valid certificate with a sane CN and dates. Some receivers score TLS quality. More importantly, broken TLS can cause delivery routing to downgrade or fail weirdly.

Decision: If expired or mismatched, fix it immediately—this is easy reputation hygiene.

Task 13: Detect backscatter (are you sending bounces to forged senders?)

cr0x@server:~$ sudo grep -E "postfix/bounce|postfix/local" /var/log/mail.log | grep -i "Undelivered Mail Returned to Sender" | tail -n 10
Jan 03 10:08:44 mx1 postfix/bounce[21201]: 9FEEA12345: sender non-delivery notification: 0ABCD12345
Jan 03 10:08:44 mx1 postfix/local[21202]: 0ABCD12345: to=<random.person@foreign-domain.tld>, relay=local, delay=0.1, dsn=2.0.0, status=sent (delivered to mailbox)

Meaning: You are generating NDRs. The key question is: are these NDRs going to legitimate senders, or to forged addresses? If your system accepted mail it shouldn’t have (open relay, no recipient validation), you’ll backscatter.

Decision: If backscatter exists, tighten SMTP inbound policy and recipient validation. Backscatter can trigger 554 rejections elsewhere because you’re now “that server.”

Task 14: Check if your server is an open relay (the fastest way to ruin everything)

cr0x@server:~$ sudo postconf -n | grep -E 'smtpd_recipient_restrictions|mynetworks|smtpd_relay_restrictions'
mynetworks = 127.0.0.0/8 [::1]/128 10.10.0.0/16
smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
smtpd_recipient_restrictions = reject_non_fqdn_recipient, reject_unknown_recipient_domain

Meaning: reject_unauth_destination is present: good. If it’s missing, you might be relaying mail for strangers.

Decision: If relay restrictions are wrong, fix immediately and rotate any exposed credentials. Assume compromise until proven otherwise.

Task 15: See the top sending accounts/apps (who is generating mail)

cr0x@server:~$ sudo grep "sasl_username=" /var/log/mail.log | awk -F"sasl_username=" '{print $2}' | awk '{print $1}' | sort | uniq -c | sort -nr | head
  902 app-prod@example.com
  117 crm-sync@example.com
   44 legacy-batch@example.com

Meaning: One principal is responsible for most traffic. If that principal is compromised or misconfigured, you’ve found your fire.

Decision: Rate-limit or suspend the top offender; require credential rotation and investigate its send patterns.

Task 16: Rate-limit Postfix outbound concurrency (stabilize behavior)

cr0x@server:~$ sudo postconf -e "default_destination_rate_delay = 2s"
cr0x@server:~$ sudo postconf -e "smtp_destination_concurrency_limit = 5"
cr0x@server:~$ sudo systemctl reload postfix
cr0x@server:~$ sudo postconf -n | grep -E "default_destination_rate_delay|smtp_destination_concurrency_limit"
default_destination_rate_delay = 2s
smtp_destination_concurrency_limit = 5

Meaning: You’ve reduced burstiness. Providers often treat smoother traffic as less suspicious, and you reduce collateral damage while you repair reputation.

Decision: Keep limits while you remediate. Later, raise cautiously based on acceptance rates and complaint signals.

Three corporate mini-stories (how teams get burned)

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

A mid-sized SaaS company moved outbound mail from a managed provider to “our own Postfix” to save costs. The assumption was simple and sincere: “Email is just SMTP; if it leaves our box, we’re done.” They updated DNS for SPF. They even published a DKIM key. Then the 554s started—first on password resets, then invoices, then onboarding.

The on-call team chased content. They removed links. They rewrote subject lines. They made the emails uglier, which is an achievement. Rejections continued. The real issue lived in the invisible plumbing: they were sending with From: example.com, but their bounce domain (envelope MAIL FROM) was a vendor subdomain used by a legacy system. SPF was passing for the bounce domain, DKIM was passing for a different domain, and DMARC was failing alignment. To the big providers, this looked like brand spoofing wearing a clever disguise.

Once they pulled headers from a few accepted test messages and compared them to the rejected stream, the pattern popped: the marketing platform was aligned; the app mailer wasn’t. They fixed the envelope sender to align with the From domain, re-signed DKIM with the right d=, and updated DMARC to strict only after verifying all senders.

The reputational dent didn’t vanish instantly, but the bleeding stopped within a day. The lesson stuck: deliverability isn’t “email leaves the server.” It’s “email arrives with a coherent identity.”

Mini-story 2: The optimization that backfired

A retail company had a solid reputation and a boring, stable outbound IP. Then Black Friday approached and someone did the sensible-sounding optimization: increase concurrency and remove rate delays so “the queue clears faster.” The MTA went from a steady stream to a firehose. Campaign sends finished earlier. Everyone high-fived.

Within hours, Microsoft domains started responding with 554 and 5.7.1 variants. Gmail began deferring and then rejecting. Internal dashboards showed “sent successfully” because the app only tracked handoff to the MTA. Customers didn’t see receipts. Support channels lit up. Finance noticed before engineering did, which is always a fun way to start your day.

The real damage wasn’t just volume—it was shape. Providers saw a sudden change in behavior: same sender, same content family, but radically different rate and concurrency. Their models treated it like a compromised sender trying to blast before it gets shut down. The company had accidentally simulated an attacker.

Rolling back concurrency helped, but reputation recovery took longer because the send burst triggered complaint spikes (“why am I getting so many emails at once?”). The fix was unglamorous: reinstate rate limits, segment traffic by importance, and schedule marketing waves to behave like a mature system, not a panicked intern with a megaphone.

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

A payments company ran two outbound paths: one for marketing and one for transactional events. It felt redundant. It cost a bit more. It also prevented a whole category of outages.

One afternoon, a CRM integration imported a dirty list. Bounces spiked, and within hours, the marketing IP started seeing blocks. The transactional stream—password resets, receipts, chargeback notices—kept delivering because it used a different IP, different envelope sender, and a stricter allowlist of internal systems that could submit mail.

Because they were disciplined, they also had DMARC reports going to a monitored mailbox and a small script that summarized pass/fail rates. The team spotted an increase in misaligned mail early and paused the integration before the provider feedback loops got angry. They cleaned the list, fixed the integration’s submission path, and resumed with a warmup schedule.

Nobody wrote a postmortem that went viral. That’s the point. The practice was dull, correct, and quietly profitable.

Common mistakes: symptoms → root cause → fix

1) Symptom: 554 only at Gmail, everything else mostly OK

Root cause: Gmail is particularly sensitive to domain reputation, engagement, and alignment. Often it’s a new IP/domain warmup issue or poor list hygiene.

Fix: Slow down sends, segment to most engaged recipients first, verify DMARC alignment, remove dormant/unverified addresses, and keep transactional traffic clean and steady.

2) Symptom: 554 starts right after moving to a new server/IP

Root cause: Cold IP reputation. Providers have no positive history for you, and your traffic looks like a bulk sender or compromised machine.

Fix: Warm up: start with low volume, high-quality recipients, gradual ramp, stable identity (PTR/HELO/TLS), and avoid sudden bursts.

3) Symptom: SPF passes but DMARC fails in headers

Root cause: SPF pass is for the envelope sender domain, not aligned with the From domain; or DKIM signs with a different domain.

Fix: Align identities: make DKIM d= match the From domain, or align MAIL FROM domain with From domain and ensure SPF authorizes the sending IP.

4) Symptom: Microsoft domains reject with 554/5.7.1 after a marketing blast

Root cause: Complaint rate and bursty traffic. Microsoft is sensitive to sudden shifts and “campaign-like” behavior from infrastructure that normally sends transactional.

Fix: Separate streams. Don’t send marketing from transactional infrastructure. Implement per-domain throttling and monitor complaint/bounce signals.

5) Symptom: Many bounces to non-existent recipients, then blocks everywhere

Root cause: Bad list hygiene or a compromised sender enumerating addresses. High invalid recipient rates are a strong spam signal.

Fix: Stop sending to unknowns. Suppress hard bounces immediately. Require double opt-in or verified addresses for bulk mail. Investigate compromise.

6) Symptom: Rejections say “HELO/EHLO” or “hostname” issues

Root cause: HELO name not resolvable, PTR missing, or mismatch between HELO and reverse DNS.

Fix: Set myhostname to a DNS name with A record matching the sending IP and ensure PTR points back. Keep it stable.

7) Symptom: Suddenly you’re “sending spam” but you didn’t change email

Root cause: Someone else changed behavior: a new integration, a retry loop, a queue replay, or a credential leak.

Fix: Identify top submitters, freeze suspicious accounts, rotate credentials, and review mail logs for patterns (destinations, rates, errors).

8) Symptom: “554 spam detected” after you enabled a new tracking or link shortener

Root cause: URL reputation. Some redirect domains are abused heavily, and your content inherits that stink.

Fix: Use branded tracking domains with solid DNS/auth; avoid sketchy shorteners; ensure landing domains have good reputation and TLS.

Checklists / step-by-step plan (get back to inbox)

Phase 0: Stop the bleeding (first 1–2 hours)

1. Split critical vs non-critical mail. Pause marketing/bulk. Keep transactional only if it’s clean. If you can’t split, pause everything and use an alternate, trusted provider temporarily.
2. Rate-limit outbound. Your goal is “predictable, low complaint” not “fast.” Implement destination rate delays and concurrency limits.
3. Stop retrying hard bounces. Treat 5xx as terminal. Retrying 5xx makes you look like a machine that doesn’t learn.
4. Confirm identity signals are coherent. PTR, HELO, TLS cert, SPF, DKIM, DMARC alignment. Fix obvious breaks immediately.

Phase 1: Triage evidence (same day)

1. Extract a sample of rejected logs and group by provider and rejection string.
2. Pull headers from a successfully delivered message to each major provider (seed accounts help). Compare auth results.
3. Compute bounce rate and hard bounce list. If you’re above a low single-digit percent on bulk, assume you have list trouble. For transactional, hard bounces should be rare.
4. Identify top submitters and code paths. Look for one offender: a job gone wild, a loop, a compromised token, or a vendor misrouting mail.

Phase 2: Repair identity (1–3 days, often faster)

1. Fix SPF to be correct and minimal. Authorize only real senders. Keep DNS lookups under limits by pruning nested includes.
2. Standardize DKIM signing. One or two selectors, rotate keys on a schedule, ensure all systems sign with aligned d=.
3. Use DMARC intentionally. If you don’t already have it, start with p=none for reporting and move to quarantine/reject after alignment is proven. If you already have reject, don’t panic-relax unless you must.
4. Stabilize infrastructure identity. rDNS, HELO, consistent outbound IPs, and sane TLS.

Phase 3: Repair reputation (days to weeks, but you can shorten it)

1. Warm up like you mean it. Start with engaged recipients. Gradually increase volume. Avoid spikes and sudden campaigns.
2. Clean lists aggressively. Suppress hard bounces immediately. Remove dormant addresses from bulk sends. Prefer confirmed opt-in for new adds.
3. Separate streams permanently. Transactional mail should not share IP/domain behavior with marketing blasts. If you must share a domain, at least segment by subdomain and signing keys.
4. Instrument deliverability like production. Track acceptance rate, bounce reason distributions, per-provider outcomes, and queue delays. “We sent it” is not a success metric.

Phase 4: Prevent recurrence (ongoing)

1. Change management for email. Concurrency, retry policies, new integrations, template changes—these are production changes. Review them.
2. Abuse and compromise monitoring. Alert on volume anomalies, new destination domains, and spikes in invalid recipients.
3. Key rotation runbook. DKIM rotation should be routine, not a once-a-year outage generator.

A reliability quote that applies

Hope is not a strategy — paraphrased idea often used in engineering and operations circles. Apply it to email, too.

FAQ

1) Is “554 spam detected” always about content?

No. Content can trigger filters, but most “sudden 554” events are identity or behavior: failed alignment, cold IP, bounce spikes, or bursty traffic.

2) Should I switch to a new IP immediately?

Only if you can also fix the underlying behavior. A new IP with the same bad list, same misalignment, or same burst pattern will get burned again—often faster.

3) Can I “warm up” by sending to every address I have?

That’s not warmup; that’s reputation arson. Warmup works when recipients are real, engaged, and expecting your mail. Start with the best cohort.

4) What’s the difference between SPF pass and DMARC pass?

SPF pass means the sending IP is authorized for the envelope sender domain. DMARC pass means SPF or DKIM passed and aligns with the visible From domain.

5) Why do I see “in reply to end of DATA command” in the rejection?

It means the receiver waited until it saw headers/body (and sometimes ran content + reputation checks) before rejecting. That points you toward policy/reputation/content, not basic SMTP connectivity.

6) If I have DMARC p=reject, will that cause 554 rejections?

It can—if your messages fail alignment. DMARC reject doesn’t cause rejection by itself; failing DMARC does. The fix is aligning senders, not deleting DMARC.

7) How do I protect transactional email when marketing gets blocked?

Separate infrastructure and identities. At minimum: separate IPs and submission credentials. Prefer separate subdomains and DKIM selectors. Keep transactional volume steady and clean.

8) Why do some recipients get mail while others bounce with 554?

Providers personalize filtering. Engagement history, user-level rules, and local policies vary. That’s why you need domain-level telemetry and seed testing, not anecdotes.

9) Should I lower DKIM key size to “improve deliverability”?

No. Use modern, secure signing (typically 2048-bit RSA where supported). Deliverability doesn’t improve by weakening crypto; it improves by being consistent and trustworthy.

10) How long does reputation recovery take?

If the issue is broken authentication or obvious misconfig, you can improve acceptance the same day. If you burned reputation with bad lists or volume spikes, expect days to weeks, depending on how disciplined you are.

Conclusion: next steps that actually move the needle

If you’re staring at “554 spam detected,” your fastest win is not a new template or a new subject line. It’s coherent identity plus predictable behavior:

1. Use the fast diagnosis playbook: scope → alignment → reputation/behavior → content.
2. Fix SPF/DKIM/DMARC alignment so the From domain is authenticated the way receivers expect.
3. Stabilize infrastructure identity: PTR, HELO, TLS, consistent egress IP.
4. Rate-limit and segment traffic. Pause bulk while you protect transactional.
5. Clean lists like your job depends on it—because it does.

Do those, and you stop wasting weeks arguing with mail filters. You’ll still have to earn trust back, but you’ll be earning it deliberately—not accidentally digging deeper.