SMTP 550 error: what it means and how to fix it

Quick answer for smtp 550 error

RFC 5321 defines 550 as the mailbox or action is not available, permanent. Enhanced codes like 5.7.1 add policy flavor. Your runbook should parse product 550s separately from destination 550s.

MailerZ outbound: Free has no send-as. Paid but wrong From: 550. Credentials work and From is still wrong: still 550. Auth success is not authorization.

MailerZ inbound: unknown recipients on a reject policy, or leftover paths that never hit us. Empty history plus a sender screenshot of 550 might be the old host, not us.

Fixes are specific: upgrade and authorize From; create the alias; delete leftover MX; stop using php mail; respect hourly caps if the text says so. There is no universal 550 toggle.

Authoritative mail transport is still IETF RFC 5321 — Simple Mail Transfer Protocol. troubleshooting. delivery recovery. docs.

The real decision

People google “550” and apply the first Gmail help page to a MailerZ refuse. Wrong universe.

Decision: who emitted the 550 — MailerZ or the destination? History tells you. Sender screenshots without our hop are often the old MX.

551–554 are neighbors, not synonyms. Do not reuse this article’s fix list on a 421.

A suite seat will 550 for its own reasons (user unknown, mailbox full). Buying one does not decode our 5.7.1.

When this path is enough

• You have the full SMTP line.
• You know which hop said 550.
• You can change plan, From, alias, or MX based on that line.

When this is the wrong ticket

• You only have “it bounced.”
• You want 550 to become a retry (that is 4xx).
• You will dual-MX as a workaround.

Technical mail flow

Outbound: client AUTH, MAIL FROM/From identity check, then relay. Failure at identity is 550 before DKIM.

Inbound: sender to MX. If leftover MX, the 550 may come from Google for a user that does not exist there.

Destination 550 after we forwarded: our history may show the destination refusal. That is their policy, not missing SPF on your zone.

Caps: exceeding hourly send-as is a refuse, not a filter. Read the text.

Step-by-step decision path

1. Paste the full line. Including 5.x.x if present.
2. Name the emitter. MailerZ history versus destination.
3. If unauthorized From, authorize or stop. Free cannot send.
4. If unknown recipient inbound, create alias or accept HOLD. Catch-all FORWARD is paid and optional.
5. If leftover MX, delete it. Two resolvers.
6. If destination policy, take headers to that admin. Do not edit our SPF for their 550.
7. Re-probe from another mailbox. Self-send lies.

Worked examples

Outlook submitted as a teammate’s address. MailerZ 550 5.7.1. They switched From to the approved alias. Mail flowed. SPF was never touched.

WordPress used php mail as hello@. Destination 550 spam. Not our 550. Pointing the plugin at SMTP then failed Free. Upgrade fixed the second 550. Two different 550s in one week.

Sender got 550 from aspmx because leftover Google had no such user. History empty. Unmix MX. The “MailerZ 550” was Google.

Agency burst 40 send-as in an hour on Starter. 550 after 10. They queued. Not DNS.

User unknown on a named alias typo. Creating the alias cleared inbound 550 from us if we were rejecting that path. HOLD would have stored instead of 550 depending on policy — know which you configured.

Failure modes and proof

MailerZ workflow and product boundary

Secuno LLC operates MailerZ. Site: mailerz.net. App: mail.mailerz.net. Envelope SRS only. Header From, Subject, Date, Message-ID, body, and MIME are never rewritten. Not IMAP. Not an open relay.

What MailerZ does

• Accept MX for verified domains.
• Rewrite envelope MAIL FROM with SRS on the forward.
• Leave Header From and MIME intact.
• Hold unknowns on Free. Optional paid FORWARD.
• Paid SMTP from approved identities. Dashboard SPF, DKIM, and DMARC instructions.
• 550 for unauthorized From. Not an open relay.

What MailerZ does not do

• Guarantee Gmail Primary or any inbox placement rate.
• Host IMAP, Calendar, or a Workspace suite.
• Send-as on Free.
• SOC 2, ISO 27001, HIPAA, review counts, or an uptime SLA. Controls: Security and Trust Center.

Plans: pricing. Free $0, 1 domain, 10 aliases, 1 seat, 14-day store, send-as disabled, SMTP and API disabled. Solo $40/year, 5 domains, 25 aliases, 90-day, 2,500 outgoing, 20/hour. Starter $8 or $80, 8/50/5, 5,000, 40/hour. Business $19 or $190, 25/200/25, 12,000, 60/hour. Agency $39 or $390, 100/500/50, 20,000, 60/hour. Unlimited is $99/month or $990/year. Annual Starter, Business, and Agency include two months free versus monthly. Solo is yearly only.

Cost, alternatives, and trade-offs

Field notes

Permanent means do not retry the same message all night. Fix the cause, then send a new message.

Store the 550 with the message id. 14/90 day windows.

Related: /troubleshooting, /delivery-recovery, /docs, /send-reply.

551, 552, 553, 554 have their own article. Do not collapse them.

Open relay refusal is a feature. Do not ask support to “just send it.”

Gmail confirmation 550s during send-as setup are often leftover MX eating the confirm.

No inbox SLA hides inside a destination 550 either.

Security: do not paste SMTP passwords with the 550 line.

Quarterly: re-read dashboard 550 wording if we clarify texts.

Teaching phrase: “550 is a sentence. Read the rest of the sentence.”

Search demand for smtp 550 error usually arrives as a screenshot, not a sentence. Ask for the received copy, the public MX set, and the plan name before you change a record. SMTP 550 Error: What It Means and How to Fix It is a workflow, not a checkbox.

Write a one-paragraph policy the team can reuse for smtp 550 error. Name the hop (inbound versus outbound), the proof artifact, and the thing you will not do (dual MX, From rewrite, second SPF record). Put the paragraph in the ticket template.

Change control matters more than a clever record. One person owns DNS for this domain. Adding an include, a selector, or a backup MX requires a ticket. Most regressions in smtp 550 error are Friday edits without a probe.

Self-send remains invalid for smtp 550 error. Gmail can short-circuit. Outlook can look local. Use a mailbox on another provider and a unique subject. If the customer refuses, the ticket stays open.

Leftover MX masquerades as every authentication and spam incident. If history is empty, smtp 550 error is the wrong title until two resolvers agree on one product. Delete aspmx, Microsoft, and registrar MX. Wait TTL.

Free HOLD and missing aliases masquerade as outages. History shows the hold. Create the named local-part or accept that unknowns stay. Catch-all FORWARD is paid, optional, and a spam trade-off — not a debugger for smtp 550 error.

Paid send-as is a different hop from inbound smtp 550 error. Free cannot send. Unauthorized From is 550 / 550 5.7.1. Publishing prettier DNS will not authorize a From the product has not approved.

Destination filters still win after a clean hop. SMTP 550 Error: What It Means and How to Fix It does not include an inbox placement SLA, review counts, or a Primary guarantee. Say that once, early, so the customer stops buying a story you cannot ship.

Proof packet for smtp 550 error: public MX from two resolvers, inbound received copy with Header From, Authentication-Results, outbound received copy if they send, plan name, and the SMTP line if anything refused. That packet ends folklore.

Retention is 14 days on Free and 90 on paid. Export headers while they live. smtp 550 error arguments without artifacts become myths. The destination inbox is the archive, not the hop store.

Agencies should not blend clients in one smtp 550 error thread. One domain, one matrix, one MX screenshot. Agency plan limits are 100 domains, 500 aliases, 50 seats, 20,000 outgoing, 60/hour — still not unlimited, still not an SLA.

Security hygiene: no SMTP passwords in the smtp 550 error ticket, no message bodies in chat, no invented SOC 2. Controls live on the Security and Trust Center. Secrets rotate if they leaked.

Related operations stay on real routes: forwarding, send-as, troubleshooting, tools, delivery recovery, docs, pricing. Do not invent a pillar. If smtp 550 error is actually leftover MX, say leftover MX.

Refresh cadence is at least quarterly, sooner after a domain transfer, panel change, ESP trial, or dashboard host change. SMTP 550 Error: What It Means and How to Fix It goes stale when the zone changes and the runbook does not.

If two products still share MX after you explained the coin flip, stop adding records. Exclusive MX is a hard stop. smtp 550 error cannot be correct on a split path.

If Header From is already rewritten, stop tuning SPF for smtp 550 error. Change the hop. MailerZ will not offer a From-replace control. Honest identity is the product.

If the customer wants a suite, sell the suite as Calendar and a hosted mailbox, not as a magic fix for smtp 550 error. Workspace and Microsoft 365 are different architectures with their own filters and their own 550s.

Hourly and monthly send-as ceilings (disabled/2,500/5,000/12,000/20,000 outgoing; 20/40/60/60 per hour by plan) produce refuses that look like smtp 550 error outages. Read counters before you republish DKIM.

Null MX plus a real MX is another lie. Remove the lone-dot refuse if you intend to receive. Empty inbound plus a pretty smtp 550 error essay is still a refuse-all.

After you change anything, wait TTL, probe from another mailbox, and store the new received source next to the MX screenshot. SMTP 550 Error: What It Means and How to Fix It is done when proof exists, not when the panel is green.

Field story 1 for smtp 550 error: Outlook submitted as a teammate’s address. MailerZ 550 5.7.1. They switched From to the approved alias. Mail flowed. SPF was never touched. Keep that story in the runbook so the next person does not reopen SMTP 550 Error: What It Means and How to Fix It from zero.

Field story 2 for smtp 550 error: WordPress used php mail as hello@. Destination 550 spam. Not our 550. Pointing the plugin at SMTP then failed Free. Upgrade fixed the second 550. Two different 550s in one week. Keep that story in the runbook so the next person does not reopen SMTP 550 Error: What It Means and How to Fix It from zero.

Field story 3 for smtp 550 error: Sender got 550 from aspmx because leftover Google had no such user. History empty. Unmix MX. The “MailerZ 550” was Google. Keep that story in the runbook so the next person does not reopen SMTP 550 Error: What It Means and How to Fix It from zero.

Field story 4 for smtp 550 error: Agency burst 40 send-as in an hour on Starter. 550 after 10. They queued. Not DNS. Keep that story in the runbook so the next person does not reopen SMTP 550 Error: What It Means and How to Fix It from zero.

Field story 5 for smtp 550 error: User unknown on a named alias typo. Creating the alias cleared inbound 550 from us if we were rejecting that path. HOLD would have stored instead of 550 depending on policy — know which you configured. Keep that story in the runbook so the next person does not reopen SMTP 550 Error: What It Means and How to Fix It from zero.

For smtp 550 error, symptom “5.7.1 on send-as” usually means Unauthorized or Free. Isolate it with Plan + From. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Empty history 550” usually means Old MX answered. Isolate it with Public MX. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Destination 550 after hop” usually means Their policy. Isolate it with SMTP text. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Hourly cap” usually means Plan ceiling. Isolate it with Counters. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Unknown user at leftover Google” usually means Mixed MX. Isolate it with Resolvers. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “php mail 550” usually means Not our SMTP. Isolate it with Plugin host. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Auth works, still 550” usually means Identity ≠ AUTH. Isolate it with Approved From. Do not stack a second change until that proof exists.

For smtp 550 error, symptom “Self-send no 550” usually means Short-circuit. Isolate it with Other mailbox. Do not stack a second change until that proof exists.

Setup step “Paste the full line.” for smtp 550 error: Including 5.x.x if present. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “Name the emitter.” for smtp 550 error: MailerZ history versus destination. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “If unauthorized From, authorize or stop.” for smtp 550 error: Free cannot send. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “If unknown recipient inbound, create alias or accept HOLD.” for smtp 550 error: Catch-all FORWARD is paid and optional. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “If leftover MX, delete it.” for smtp 550 error: Two resolvers. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “If destination policy, take headers to that admin.” for smtp 550 error: Do not edit our SPF for their 550. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Setup step “Re-probe from another mailbox.” for smtp 550 error: Self-send lies. Skip it and SMTP 550 Error: What It Means and How to Fix It turns into a second ticket next week. Do the step, store the artifact, then continue.

Trade-off on smtp 550 error: choosing Read the line, fix that cause gets you Durable and gives up Requires history. Write that exchange on the quote so nobody pretends it is free.

Trade-off on smtp 550 error: choosing Add SPF blindly gets you Busywork and gives up 550 unchanged. Write that exchange on the quote so nobody pretends it is free.

Trade-off on smtp 550 error: choosing Dual MX gets you Random 550s and gives up Worse proof. Write that exchange on the quote so nobody pretends it is free.

Trade-off on smtp 550 error: choosing Suite seat gets you Different 550s and gives up Price. Write that exchange on the quote so nobody pretends it is free.

SMTP 550 Error: What It Means and How to Fix It is enough when You have the full SMTP line. If that condition is false, stop implementing smtp 550 error and reopen the decision.

SMTP 550 Error: What It Means and How to Fix It is enough when You know which hop said 550. If that condition is false, stop implementing smtp 550 error and reopen the decision.

SMTP 550 Error: What It Means and How to Fix It is enough when You can change plan, From, alias, or MX based on that line. If that condition is false, stop implementing smtp 550 error and reopen the decision.

SMTP 550 Error: What It Means and How to Fix It is the wrong ticket when You only have “it bounced.”. Name the correct system instead of forcing smtp 550 error to cover it.

SMTP 550 Error: What It Means and How to Fix It is the wrong ticket when You want 550 to become a retry (that is 4xx). Name the correct system instead of forcing smtp 550 error to cover it.

SMTP 550 Error: What It Means and How to Fix It is the wrong ticket when You will dual-MX as a workaround. Name the correct system instead of forcing smtp 550 error to cover it.

Write smtp 550 error in the subject line of the ticket and the layer in the first sentence. If the layer is leftover MX, say leftover MX. If the layer is HOLD, say HOLD. If the layer is a destination 550, paste the text. SMTP 550 Error: What It Means and How to Fix It stays short when the first sentence is honest.

Keep a standing calendar note for smtp 550 error: monthly external probe, leftover MX lookup, and a glance at send-as counters if you are paid. Five minutes. The outage you avoid is the Friday dual-MX restore.

When two vendors disagree about smtp 550 error, believe artifacts: two resolvers, one received copy, one history row. Do not believe the prettier admin center. Registrar dots lie. Composer UIs lie. Self-send lies.

Teach the next hire the MailerZ split before they touch smtp 550 error: envelope may change, Header From must not, Free cannot send, unknowns HOLD, leftover MX is a hard stop, no inbox SLA, no SOC 2 sticker. That speech prevents a class of tickets.

If smtp 550 error appears in an RFP, answer with published caps and hop evidence. Decline inbox-rate clauses. Decline fake certifications. Point at pricing and the Security and Trust Center. SMTP 550 Error: What It Means and How to Fix It is not a place to invent enterprise theater.

Export while the 14- or 90-day window still has the smtp 550 error hop. Future-you will not remember the SMTP sentence. The destination mailbox remains the archive. We will not grow the store because a blog asked.

Do not bundle unrelated edits with smtp 550 error. Rotating SMTP while republishing MX while enabling FORWARD is how you lose the ability to name the failure. One change, one probe, one stored copy.

If you need a suite, buy a suite for Calendar and a hosted mailbox. If you need a hop, buy a hop. SMTP 550 Error: What It Means and How to Fix It does not become Exchange because a quote stacked seats next to aliases.

FAQ

Key takeaways

• 550 is permanent. Read the text.
• Name the emitter.
• Free cannot send-as.
• Auth ≠ authorization.
• Leftover MX 550s are often Google.
• Caps are 550s too.
• Do not SPF a 5.7.1.
• History first.
• No inbox SLA.
• Not an open relay.

Conclusion and next action

SMTP 550 means a permanent no. The fix is in the enhanced text and the hop that spoke.

Next action: copy the line from history, classify emitter, then change plan, From, alias, or MX — not all four at once.

Review quarterly, or sooner if DNS hosts or dashboard instructions change. Author: MailerZ editorial, Secuno LLC.