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An email error is a failure at a particular point in the delivery chain—not one universal problem. Start with the complete bounce or error text. A normal 4xx SMTP response means the receiving system is asking for a later retry; a normal 5xx response means the message was rejected and the cause should be fixed before resending. Those are conventions, not guarantees, so the enhanced status code and the provider’s explanatory text matter most.
For Gmail, look for a message from Mail Delivery Subsystem or [email protected], often titled “Delivery status notification (failure).” Google’s guide explains where to find and interpret these notices: Gmail delivery error guidance.
Where an email can fail
Delivery can break before the message leaves your device, while servers are routing it, when the recipient’s server evaluates it, or after acceptance but before inbox placement.
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- Mail app or SMTP submission: incorrect password, OAuth failure, server, port, TLS mode, or an unauthorized relay.
- Routing: a misspelled recipient domain, missing MX records, DNS failure, or a temporary network outage.
- Recipient-server acceptance: nonexistent mailbox, full storage, message-size limit, authentication failure, spam policy, or sender reputation.
- Inbox placement: spam, quarantine, user rules, forwarding, moderation, or provider suppression can prevent an accepted message from appearing in Inbox.
A message shown in Sent Items has left the local Outbox; it is not proof of inbox delivery. Some providers also suppress or discard suspicious mail without presenting a useful sender-visible error.
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How to read a bounce message
- Find the final recipient. Confirm which address actually failed, especially when a message had several recipients.
- Identify the remote server. The organization named in the diagnostic usually owns the policy that rejected or deferred the message.
- Record the complete SMTP response. Keep the first reply, such as
550 5.1.1, and all following text. - Read the enhanced status code. Its class, subject, and detail narrow the problem.
- Inspect authentication results. Look for
spf=,dkim=,dmarc=,smtp.mailfrom,header.from, and the DKIMd=value. - Save IDs and timing. The Message-ID, queue ID, sending IP, timestamp, and time zone help an administrator trace the transaction.
For example:
550 5.7.26 Unauthenticated email from example.com
is not accepted due to the domain's DMARC policy
This is a permanent policy rejection involving the sending domain’s authentication and alignment—not an invalid recipient address. Compare the envelope domain, visible From: domain, and DKIM signing domain.
SMTP and enhanced status codes at a glance
The first digit gives the usual operational direction. Code meanings are standardized broadly, but providers attach their own enhanced codes and text; use the complete response rather than a generic lookup alone.
| Code | Usual meaning | What to do |
|---|---|---|
2xx |
Accepted or completed | No corrective action normally needed; acceptance is not the same as inbox placement. |
4xx |
Temporary failure or deferral | Retry later with controlled backoff; investigate if it repeats. |
5xx |
Permanent rejection | Fix the stated cause before sending again. |
421, 450, 451 |
Temporary service, mailbox, policy, or rate-limit problem | Wait, avoid tight retry loops, and check whether one provider or all recipients are affected. |
501 5.5.4, 503 5.5.1 |
Invalid SMTP command, sequence, or HELO/EHLO | Correct the client or device hostname, command order, TLS, and submission settings. |
530 |
Authentication or TLS required | Use the provider’s authenticated submission service and required encryption. |
550 5.1.1 |
Recipient does not exist | Correct the address; do not keep retrying a confirmed nonexistent mailbox. |
553 5.1.2 |
Recipient domain cannot be found | Check the domain spelling and its DNS/MX configuration. |
552 5.2.2 |
Recipient mailbox is full | Contact the recipient another way and wait for storage to be freed. |
552 5.3.4 |
Message, attachment, or headers exceed a limit | Reduce the payload or send a cloud-storage link; limits vary by provider. |
550 5.7.1 |
Policy, spam, authorization, or reputation rejection | Follow the diagnostic text; check authentication, content, relay authorization, and reputation. |
550 5.7.26, 5.7.27, 5.7.30, 5.7.40 |
SPF, DKIM, DMARC, or unauthenticated-sender failure | Repair DNS records, signing, and alignment for the domain in From:. |
554 |
Transaction failed; details vary | Use the receiving provider’s full text; Google describes it as a failed transaction without additional detail. |
Google’s code references provide provider-specific examples for 421, 554, size limits, authentication, and policy responses: Google SMTP reply codes and Gmail SMTP error list.
Common errors and their fixes
550 5.1.1: the mailbox does not exist
Check for a typo, wrong domain suffix, old autocomplete entry, spaces, quotation marks, trailing punctuation, or a deleted account. Copy the failed address from the bounce, compare it character by character, and ask the recipient to confirm it through another channel. Gmail lists these formatting mistakes in its delivery guidance: Gmail delivery errors.
553 5.1.2: the recipient domain cannot be found
The domain may be misspelled, expired, missing usable MX records, or temporarily broken. Check it with:
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dig MX example.com
dig A example.com
dig NS example.com
On Windows:
nslookup -type=mx example.com
nslookup -type=ns example.com
If there is no functioning mail route, the recipient-domain owner or DNS administrator must repair it. Google documents this response at its SMTP error reference.
550 5.2.2: a full mailbox
The recipient must delete or archive mail or increase storage. In Google’s ecosystem, Gmail, Drive, and Photos can share storage. Repeated resends cannot create capacity.
552 5.3.4: an oversized message
There is no universal attachment maximum. Encoding can make a file larger in transit, and the recipient may impose a lower limit. Use a cloud link, compress suitable files, or split a package when practical. Too many attachments, oversized headers, and blocked archives can also trigger a size-related response.
421 or 451: temporary rejection or throttling
The recipient may be busy, unavailable, rate-limiting a sender, or seeing a temporary DNS, authentication, or reputation problem. Wait and retry with exponential backoff. Do not have an application retry every few seconds. If the response persists, compare results across recipient domains and inspect DNS, authentication, and sending volume.
550 5.7.1: policy or spam rejection
This broad response can indicate poor IP or domain reputation, an unauthorized relay, missing authentication, restrictive recipient policy, suspicious content, or malformed headers. Changing only the subject line is unlikely to fix a DNS or authorization problem. Google’s examples are listed at Gmail SMTP errors.
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Relay denied
A 550, 553, or “relay prohibited” message means the outgoing server does not authorize your account to deliver to that destination. Use the SMTP service belonging to the mail account, enable SMTP authentication, confirm the permitted From: address, and use the provider’s documented submission port—commonly 587 or 465. Microsoft describes common relay scenarios at its relay-error article.
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Printers, scanners, applications, and old libraries may send commands out of order, identify themselves with an invalid hostname, authenticate before TLS, or use an unsupported protocol. Configure a valid fully qualified hostname, the provider’s submission endpoint, the correct port and TLS mode, and current firmware or libraries. SMTP’s protocol requirements are defined in RFC 5321; Gmail discusses invalid server identities at its delivery-error guide.
A practical troubleshooting workflow
- Locate the failure. An Outbox stall points to the local client or network; an immediate login error points to credentials or SMTP AUTH; a later bounce points to routing, recipient policy, mailbox, or content; no bounce with no Inbox message points to filtering or quarantine.
- Validate the address. Check spelling, punctuation, autocomplete, and whether the recipient changed organizations. A live domain does not prove that a mailbox exists.
- Classify the response. Treat
4xxas normally transient and5xxas normally permanent. Then use the enhanced family:5.1.xaddressing,5.2.xmailbox storage,5.3.xtransaction or size,5.4.xrouting or limits,5.5.xprotocol,5.6.xformat, and5.7.xsecurity or policy. - Test DNS and authentication. For a custom domain, inspect SPF, DKIM, DMARC, MX, and reverse DNS before changing providers.
- Test content. Send a small plain-text message, then a small benign attachment, then progressively more complex content. This isolates size, MIME, file-type, and filtering causes.
- Check limits and reputation. Look for volume spikes, complaints, stale lists, repeated hard bounces, a new IP, or one provider-specific block.
- Assign ownership. Fix only the layer you control, then provide the full unedited diagnostic to the responsible administrator or provider.
Custom-domain email: SPF, DKIM, DMARC, DNS, and TLS
SPF
SPF lists authorized sending sources for the envelope sender (the MAIL FROM domain). Common failures include no record, multiple SPF TXT records, an omitted sending service, a record on the wrong domain, or more than 10 DNS lookups. Check:
dig TXT example.com
There should be one policy beginning v=spf1. Microsoft explains the single-record and lookup-limit rules at Microsoft email-authentication troubleshooting.
DKIM
DKIM signs the message so recipients can verify authorization and detect alteration. A missing selector, wrong public key, broken CNAME, mismatched key pair, gateway modification, or unavailable DNS record can fail validation.
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dig TXT selector1._domainkey.example.com
The selector must match the one in the DKIM-Signature header.
DMARC and alignment
DMARC passes when SPF or DKIM passes and aligns with the visible From: domain. Thus spf=pass can coexist with dmarc=fail if smtp.mailfrom belongs to a different domain. Check:
dig TXT _dmarc.example.com
A record normally begins v=DMARC1;. Do not move straight to p=reject until legitimate sending sources and reports are understood; an incomplete inventory can block real mail. Microsoft’s alignment and troubleshooting details are at its authentication guide.
Reverse DNS and TLS
Some providers defer or reject connections when the sending IP has no consistent PTR record, the host presents an invalid name, or required TLS is absent. Gmail lists PTR and TLS-related responses at its current error catalog. TLS encrypts the connection; it does not replace SPF, DKIM, or DMARC, which address authorization, signing, and alignment.
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Application and website email failures
Websites sending receipts, alerts, and password resets need more than a mailbox password.
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- Use authenticated SMTP submission or an API, with documented port and TLS requirements. Older devices may not support modern OAuth.
- Parse the SMTP response and classify transient versus permanent failures.
- Retry transient failures with exponential backoff and an idempotency key so one event cannot create duplicates.
- Record provider message IDs, timestamps, recipient, response text, and sending IP; remove credentials from logs.
- Process bounce and complaint webhooks. Suppress confirmed hard bounces so the application does not repeatedly send to invalid addresses.
- Keep transactional and marketing traffic appropriately separated. A marketing platform may not be suitable for password resets, while a raw transactional relay may lack consent and unsubscribe features.
A consumer mailbox is usually a poor application-sending tool because it lacks reliable event logs, webhooks, retry controls, and suppression management.
Edge cases that change the diagnosis
Forwarding and mailing lists
Forwarding commonly breaks SPF because the forwarder’s IP is not in the original SPF record. DMARC can still pass when aligned DKIM survives, but a forwarding or list system that modifies the body may break DKIM as well. ARC or a trusted intermediary configuration can help legitimate forwarded mail; see Microsoft’s forwarding guidance.
One recipient works while another fails
This usually points to a recipient-specific policy, mailbox, MX path, block, or reputation decision. It does not prove that the sender’s entire system is healthy.
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Check HELO/EHLO, port, TLS version, SMTP AUTH compatibility, OAuth support, and whether the device is attempting direct delivery instead of authenticated submission.
Internationalized addresses and domains
Non-ASCII addresses require support from every relevant client, device, DNS system, and provider. Do not label one syntactically unusual address invalid without checking standards support, domain DNS, and mailbox existence.
Who can fix the problem?
| Problem | Usually responsible |
|---|---|
| Typo or nonexistent address | Sender or recipient |
| Full recipient mailbox | Recipient |
| Broken MX or domain DNS | Recipient-domain administrator |
| SPF, DKIM, or DMARC | Sending-domain administrator |
| SMTP password, OAuth, or account settings | Sender or mailbox administrator |
| IP reputation, volume, or list quality | Sender, host, or sending provider |
| Recipient organization’s block policy | Recipient administrator |
| Provider outage | Email provider |
When to contact support
Send the administrator or provider the complete unedited bounce, date and time zone, sender and recipient domains, Message-ID or queue ID, sending IP, relevant DNS records, and a sanitized SMTP transcript. State whether the issue affects one recipient domain or many. Remove passwords, OAuth tokens, and private message content.
Should you resend or change providers?
Resend after a controlled delay for a clearly transient 4xx response, a corrected typo, or a transient connection failure. Do not immediately resend a hard bounce, full mailbox, size rejection, authentication failure, or spam block. Changing providers can improve logging, API support, or infrastructure, but it cannot repair an invalid address, broken DNS, poor consent practices, misaligned domains, or a recipient organization’s policy.
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Quick Recap
Incident checklist
- Preserve the full bounce and headers.
- Identify the final recipient and remote provider.
- Classify the first digit and enhanced status family.
- Verify the address and domain independently.
- Check mailbox capacity, message size, and attachment type.
- For custom domains, verify one SPF record, DKIM selector, DMARC alignment, MX, PTR, and TLS.
- Stop tight retry loops and suppress confirmed hard bounces.
- Check content, complaint rate, volume, and provider-specific reputation.
- Give the right administrator the timestamp, IDs, IP, DNS data, and complete diagnostic.
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