Klaviyo Bounce Codes Explained: The SMTP Error Reference for Deliverability Debugging
The SMTP status code in your Klaviyo bounce log tells you exactly what failed and which fix applies. This is the reference.
Klaviyo bounce codes are standard SMTP status codes returned by receiving mail servers. The first digit tells you whether the failure is temporary (4xx, will retry) or permanent (5xx, hard bounce). The next two digits classify the error by category: user not found, authentication failure, reputation block, mailbox full, policy rejection. The most common Klaviyo bounce codes to recognise are 550 5.1.1 (user not found, list hygiene), 550 5.7.1 (policy or blocklist block), 550 5.7.26 (DMARC alignment failure), 421 4.7.0 (reputation deferral), and Microsoft-specific S3150. Identifying the code category tells you whether the cause is list, authentication, or reputation.
Klaviyo's bounce log shows an SMTP status code for every failed delivery. Most senders ignore the codes and look only at the aggregate bounce rate. That is a missed diagnostic opportunity. The code tells you what kind of failure occurred, and different codes point to different root causes. A bounce report dominated by 5.1.1 codes is a list hygiene issue. A report dominated by 5.7.26 codes is an authentication problem. Treating them the same means applying the wrong fix.
This article is a reference for the Klaviyo bounce codes you are most likely to encounter, what each actually means, and the specific fix for each category.
How to Read an SMTP Bounce Code
SMTP status codes are three digits:
First digit: outcome category.
2xx- success (message delivered).4xx- temporary failure (retry later).5xx- permanent failure (reject, do not retry).
Second digit: subject category.
0- syntax.1- information.2- connection.3- unspecified.4- mail system (typically transient infrastructure).5- mail server (permanent infrastructure).7- policy or security.
Third digit: specific status within the subject category.
When you read a Klaviyo bounce code like 550 5.7.26, the 550 is the basic response and 5.7.26 is the Enhanced Status Code that provides more detail. The 5 indicates permanent failure, the 7 indicates a policy issue, and the 26 narrows it to DMARC alignment specifically.
The Klaviyo Bounce Codes You Need to Know
List Hygiene Codes (5.1.x family)
550 5.1.1 - User not found. The email address does not exist at the receiving domain. Clean list hygiene issue.
550 5.1.2 - Domain not found. The domain portion of the address does not resolve. Either the domain has been abandoned, or the address was mistyped at signup.
550 5.1.10 - Address rejected by filter. The address exists but is being filtered from accepting mail. Can be transient or permanent.
What to do: suppress the affected addresses. Audit recent list acquisitions for patterns. Implement signup-time email validation.
Authentication Codes (5.7.26, 5.7.27)
550 5.7.26 - DMARC alignment failure. The receiving server checked DMARC and the message failed alignment (DKIM or SPF did not align with the From domain). Under a DMARC policy of p=quarantine or p=reject, the message is rejected.
550 5.7.27 - No DKIM signature. The message lacked a DKIM signature entirely. Usually indicates a Klaviyo configuration problem where DKIM signing was not applied to this campaign or sending path.
What to do: verify Klaviyo DKIM CNAMEs in your DNS, check DMARC alignment in test messages, confirm the sending subdomain is verified in Klaviyo.
Policy and Blocklist Codes (5.7.1, 5.7.9, 5.7.511)
550 5.7.1 - Message rejected by policy. Broad category. The receiving server decided to reject based on some policy: reputation, blocklist, content, or specific sender characteristics. The accompanying text often explains which.
554 5.7.9 - Yahoo/AOL policy rejection. Yahoo-specific. Usually DMARC or authentication-related on Yahoo's infrastructure.
550 5.7.511 - Microsoft reputation or complaint block. Microsoft-specific variant. Indicates that reputation, complaint signals, or IP-level flags caused the rejection.
What to do: read the full rejection message (Klaviyo usually preserves it) to identify the specific policy cited. For reputation-based rejections, suppress low-engagement subscribers, verify authentication, and escalate to Klaviyo deliverability for IP-level signals.
Deferral Codes (4.x.x family)
421 4.7.0 - Reputation-based deferral. The receiving server is asking Klaviyo to retry later. Often a leading indicator of developing reputation problems.
421 4.3.0 - Temporary system problem. Receiving server has an infrastructure issue. Usually transient.
450 4.2.2 - Mailbox full. The specific mailbox is temporarily unable to accept mail. Usually a dormant address.
451 4.3.0 - Message processing error. Temporary, receiving server could not process.
452 4.3.1 - System resources exhausted. Temporary, receiving server is under load.
What to do: most deferrals resolve through Klaviyo's retry logic. Watch for sustained patterns at a single ISP (especially 4.7.0) as early warning of reputation issues.
Microsoft-Specific Codes
S3150 - Microsoft reputation throttle or block. Appears in the extended error message on Microsoft-originated bounces. Recoverable with clean sending over two to four weeks.
S1045 - Microsoft-specific authentication failure.
S3110 - Microsoft spam content detection.
What to do: Microsoft-specific recovery requires engagement-focused sending, authentication alignment, and potentially escalation to Klaviyo deliverability for IP-level support, because Klaviyo owns the IPs and has the only direct line to Microsoft Sender Support on IP matters.
Relay and Forwarding Codes
550 5.4.1 - Relay access denied. Receiving server will not accept mail for this recipient. Often an authentication or configuration issue at the sending side.
550 5.4.4 - Routing error. DNS or MX configuration problem for the receiving domain.
What to do: typically address-specific. Suppress and audit for patterns if many appear.
Rate-Limiting Codes
421 4.7.26 - Too many simultaneous connections. Receiving server is rate-limiting.
452 4.5.3 - Too many recipients in one transaction. Klaviyo's sending infrastructure usually handles this automatically.
What to do: for sustained rate-limiting, review sending volume patterns. Smooth out spikes.
How to Aggregate Klaviyo Bounce Codes
Individual bounces tell you less than patterns. A practical approach to reading Klaviyo bounce data:
Step 1: Export bounce log from Klaviyo for the last 30 days.
Step 2: Group by SMTP code. Count occurrences.
Step 3: Look at the top three codes. If they are list-hygiene codes (5.1.x), the fix is list hygiene. If they are authentication codes (5.7.26, 5.7.27), the fix is DKIM and DMARC. If they are policy codes (5.7.1, 5.7.9, 5.7.511), the fix is reputation and engagement.
Step 4: Cross-reference with recipient domain. Policy codes concentrated at Microsoft point to Microsoft-specific reputation issues. List codes distributed across many domains point to general list decay. Authentication codes at Gmail point to DKIM configuration problems.
This aggregation takes 15 minutes in a spreadsheet and replaces hours of guesswork.
What the Klaviyo Dashboard Does Not Show
Klaviyo's aggregate bounce rate metric hides the code distribution. Two accounts with a 3% bounce rate can have completely different problems: one dominated by 5.1.1 codes (list hygiene), the other by 5.7.26 codes (authentication). The aggregate number is the same; the fixes are entirely different.
To see the breakdown, you have to export and aggregate bounces yourself, or use a monitoring tool that does it automatically. The Trial Audit at engagor.ai/klaviyo-trial-audit classifies your bounces by root cause and tells you which category dominates, so you know where to focus. Written report in 24-48 hours.
Get visibility before you change anything
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Frequently asked questions
What do Klaviyo bounce codes mean?
Klaviyo bounce codes are standard SMTP status codes returned by receiving mail servers. The first digit indicates outcome (2xx success, 4xx temporary, 5xx permanent). The next digits classify the error type: 5.1.x for list/address issues, 5.7.x for policy/security issues, 4.x.x for temporary failures. Each code points to a specific root cause.
What does 550 5.1.1 mean in Klaviyo?
550 5.1.1 means "user not found" - the recipient email address does not exist at the receiving domain. This is a list hygiene issue. Klaviyo suppresses the address automatically. If 5.1.1 codes dominate your bounce log, the underlying cause is invalid or decayed list data that needs validation.
What does 550 5.7.26 mean in Klaviyo?
550 5.7.26 is a DMARC alignment failure. The receiving server checked DMARC on the message, it failed, and the domain's DMARC policy told the server to reject. Fix by correcting Klaviyo DKIM alignment: verify the CNAME records point to Klaviyo's DKIM keys and confirm the signing subdomain aligns with your From domain.
What is the difference between 4.x.x and 5.x.x bounce codes?
4.x.x codes are temporary failures. The receiving server asks the sender to retry later. Klaviyo retries 4.x.x bounces on a schedule and most eventually deliver. 5.x.x codes are permanent failures. Klaviyo suppresses the address and does not retry. Watch 4.x.x trends as early warning for reputation issues that may escalate to 5.x.x.
What is the S3150 error code in Klaviyo?
S3150 is a Microsoft-specific error indicating that mail from the sending IP has been throttled or blocked due to reputation or complaint signals. It is not a permanent blocklist but a reputation-driven rejection that recovers with clean sending over two to four weeks.
How do I read Klaviyo bounce logs?
Export the bounce log from Klaviyo, group by SMTP code, count occurrences, and cross-reference with recipient domain. The dominant codes tell you the root cause category (list hygiene, authentication, reputation). The recipient-domain pattern tells you which ISP's rules you are hitting. Together they point to the specific fix.
Does Klaviyo show the reason for each bounce?
Yes. Klaviyo's bounce log shows the SMTP status code and the accompanying response text from the receiving server. The response text often includes specific detail about the rejection ("domain not in SPF record," "DMARC policy violated," "reputation block"). Read it before deciding what to fix.