When an email bounces, it comes with a reason. SMTP error codes — the three-digit numeric codes returned by receiving mail servers — encode exactly what happened, why the delivery failed, and in many cases, what you need to do to fix it.
Most email senders treat bounce codes as noise: a category label (‘hard bounce’ or ‘soft bounce’) in their ESPs’ reporting, without any deeper investigation. Understanding SMTP error codes, email servers return transforms bounce data from a red metric into a diagnostic tool that points to specific deliverability infrastructure problems.
The SMTP Response Code Structure
SMTP response codes follow a three-digit format: X.Y.Z
- First digit — Class: 2 = Success, 4 = Temporary failure (soft bounce), 5 = Permanent failure (hard bounce).
- Second digit — Subject: 0 = General, 1 = Addressing, 2 = Mailbox, 3 = Mail system, 4 = Network, 5 = Protocol, 6 = Content, 7 = Security.
- Third digit — Detail: Specific sub-category of the subject code.
The full bounce message typically includes the three-digit code plus a longer Enhanced Status Code (e.g., 5.1.1) and a human-readable explanation from the receiving server.
4xx Error Codes: Temporary Failures (Soft Bounces)
4xx codes indicate a temporary condition at the receiving server. The sending server should retry delivery. If the condition persists through multiple retry attempts, the message eventually bounces permanently.
421 — Service Not Available / Try Again Later
The receiving mail server is temporarily unavailable, overloaded, or rate-limiting connections from your sending IP. This is one of the most common soft bounce codes.
What it means for you: Your sending IP is being throttled by the receiving server. This can indicate:
- Your sending volume exceeded the receiving server’s per-connection limits — common for bulk sends to a single domain.
- The receiving server’s IP has you on a temporary grey list — test connections to verify new senders before accepting messages.
- Your sending domain reputation with this ISP is declining — the server is slowing delivery as a warning signal.
Action: Reduce sending frequency to this domain. Check Google Postmaster Tools for domain reputation signals. If throttling is from a major ISP (Gmail returns 421 4.7.0 for rate limiting), review your recent campaign bounce and complaint rates.
450 — Requested Mail Action Not Taken: Mailbox Unavailable
The mailbox exists but is temporarily unavailable — typically because it is full (quota exceeded) or the account is temporarily suspended.
Action: Retry after 24–48 hours. If the address continues to return 450 on multiple retries over several days, treat it as a soft-hard bounce and suppress it.
451 — Requested Action Aborted: Local Error in Processing
The receiving server encountered a temporary internal error. May also indicate that the receiving server cannot complete a required DNS lookup or authentication check.
Action: Retry automatically. If 451 persists, check your DKIM and SPF configuration — a failed authentication lookup from the receiving server’s perspective can produce this code.
5xx Error Codes: Permanent Failures (Hard Bounces)
5xx codes indicate permanent delivery failures. The address should be suppressed immediately — retrying will produce the same result.
550 — Mailbox Does Not Exist / User Unknown
The most common hard bounce code. The specific mailbox in the RCPT TO command does not exist on the receiving server. The address is invalid.
Variants:
- 550 5.1.1: User unknown. The email address does not exist. Hard suppress immediately.
- 550 5.1.2: Bad destination mailbox address. Similar to 5.1.1, typically for domain-level issues.
- 550 5.1.10: ORCP — The organisation that owns the domain does not permit sending to that address (often corporate policy blocks).
- 550 5.7.1: Message rejected due to reputation or policy. The receiving server has blocked your message based on the sending IP or domain reputation. Check blacklist status and domain reputation.
- 550 5.7.26: DMARC failure. Your email failed DMARC alignment. Check SPF and DKIM alignment for the From domain.
Action: 550 5.1.1 and 550 5.1.2 warrant immediate hard suppression. 550 5.7.1 warrants a deliverability audit — it indicates a reputation problem, not just an invalid address.
551 — User Not Local / Please Try Forward Address
The receiving server does not accept email for this address — it is directing you to a different address or server. Rare in practice.
552 — Exceeded Storage Allocation
The recipient’s mailbox is full. This returns as a 5xx (permanent) code in some server implementations, though the underlying cause is temporary. Treat as soft bounce with extended retry.
553 — Mailbox Name Not Allowed
The email address format is not permitted by the receiving server. Often indicates an invalid local part (the portion before the @).
554 — Transaction Failed
A catch-all error indicating the receiving server rejected the message for an unspecified reason. The reason is usually in the accompanying text:
- 554 5.7.1 (Message blocked): The sending IP or domain is on a blacklist used by the receiving server. Check blacklist status immediately.
- 554 5.6.0 (Message rejected — content): The email content triggered a content filter. Review for spam trigger patterns.
- 554 Reject: From domain not allowed: Your sending domain is not permitted by the receiving domain’s policy. Check if the recipient domain has strict allow-list policies.
Gmail-Specific SMTP Error Codes
Gmail (Google Workspace) returns specific error codes that require Gmail Postmaster Tools for full diagnosis:
- 550 5.7.1 — Our system has detected that this message is likely suspicious: The email is flagged by Gmail’s spam detection based on content, authentication, or sending pattern signals.
- 421 4.7.0 — Our system has detected an unusual rate of unsolicited mail: Gmail is rate-limiting your sends. Reduce volume and monitor domain reputation in Postmaster Tools.
- 550 5.7.28 — The activity of this account violated the Gmail Terms of Service: Severe spam or policy violation. Requires account-level investigation.
Microsoft 365 / Outlook SMTP Error Codes
- 550 5.7.1 — Service unavailable; client host blocked: Your sending IP is blocked by Microsoft. Use Microsoft’s delist portal at sender.office.com.
- 550 5.7.23 — The message was rejected because of Sender Policy Framework violation: Your SPF record does not include the sending IP. Update your SPF record.
- 421 4.7.0 — Temporary server error: Microsoft is temporarily throttling your sends. Common with new sending domains. Continue at lower volume.
Using SMTP Error Codes for List Management
Translating error codes into list management actions:
- 550 5.1.1, 550 5.1.2, 553: Immediate hard suppress. The address is invalid.
- 550 5.7.1 (reputation/blacklist): Deliverability audit required. Check blacklist status. Do not suppress the address — the problem is with your sending reputation, not the recipient’s address.
- 550 5.7.26 (DMARC failure): Authentication configuration fix required. All email is affected, not just this recipient.
- 421 (rate limiting): Reduce volume. Spread sends over longer time windows.
- 450 (mailbox full): Soft suppress and retry after 24–48 hours.
Key Takeaways
- SMTP error codes email servers return contain specific diagnostic information — not just bounce/no-bounce classification.
- 4xx codes are temporary failures. 5xx codes are permanent failures that require immediate suppression.
- 550 5.1.1 (mailbox does not exist) is the most common hard bounce — immediate suppression required.
- 550 5.7.1 (reputation blocked) and 550 5.7.26 (DMARC failure) indicate infrastructure problems that affect all your sends — not individual address problems.
- Gmail and Microsoft 365 return platform-specific error codes that require platform-specific monitoring tools (Postmaster Tools, SNDS) for full diagnosis.
Frequently Asked Questions
4xx errors are temporary — the sending server should retry delivery later. 5xx errors are permanent — the address should be suppressed immediately and never retried.
550 5.7.1 indicates the receiving server has blocked your message based on reputation signals — typically a blacklisting, a spam score trigger, or a DMARC policy failure. It is a serious deliverability signal that requires immediate investigation of your domain reputation, authentication configuration, and blacklist status
Most ESPs (Mailchimp, SendGrid, ActiveCampaign, Klaviyo) display bounce codes in campaign reports and bounce logs. For full SMTP transaction logs, check your email platform’s delivery log or API event history.
Conclusion
SMTP error codes that email servers return are the most specific, actionable deliverability data available to any email sender. Reading them correctly transforms your bounce report from a performance metric into a troubleshooting guide that points directly at what needs to be fixed — whether that is an invalid address, a misconfigured authentication record, a blacklisted IP, or a reputation problem that is degrading delivery across all your sends.
