An email bounce occurs when a message cannot be delivered to the recipient. Every bounce carries an SMTP response code that tells you exactly why the delivery failed and what to do about it.

This guide covers:

SMTP 4xx codes — soft bounces (temporary failures, retryable)
SMTP 5xx codes — hard bounces (permanent failures)
Enhanced delivery status codes (RFC 3463 — X.Y.Z format)
ISP-specific bounce codes (Microsoft, Gmail, Yahoo)
Bounce message interpretation
Recommended bounce rate thresholds
Relevant resources and tools

Quick decision guide

Before you look at individual codes, use this rule of thumb:

Code range	Bounce type	Retry?	List action
`4xx`	Soft bounce (temporary)	Yes	Keep the address. Remove it only if the soft bounces persist (5 or more soft bounces on the same address).
`5xx`	Hard bounce (permanent)	No	Remove the address immediately and add it to the suppression list.
`5.X.X`	Enhanced status (RFC 3463)	Depends on the subcode	Follow the code-specific action in Enhanced status codes.

SMTP 4xx: soft bounce codes (temporary)

4xx codes indicate a temporary failure. The sending server automatically retries delivery. These codes do not require immediate list cleanup, but repeated soft bounces to the same address must be treated as hard bounces after 5 or more failures.

Code	Name	What it means	Recommended action
`421`	Service temporarily unavailable	The receiving server is busy or temporarily down. The mail is retried.	The ESP retries automatically. If it persists beyond 24 hours, there may be an issue with the recipient server.
`450`	Mailbox unavailable (temporary)	The recipient mailbox is temporarily unavailable. It may be locked or over quota.	Retry later. No action is needed unless the pattern repeats.
`451`	Local error in processing	A server-side processing error, often a spam filter or policy hold.	Monitor it. If it is domain-wide, check the sending IP reputation.
`452`	Insufficient storage or quota exceeded	The receiving server or mailbox has run out of storage space.	Retry later. This is not a sender-side issue.
`455`	Server unable to accept parameters	The server cannot handle the current request parameters, due to a timing or resource issue.	The ESP retries automatically. This may indicate rate limiting or resource overload.

SMTP 5xx: hard bounce codes (permanent)

Never retry a 5xx bounce

5xx codes are permanent failures. Remove the address from your active list immediately and add it to the suppression list. Repeated sending to hard-bounced addresses damages your sender reputation significantly.

Code	Name	What it means	Recommended action
`500`	Syntax error or command unrecognised	The server could not parse the command sent. This is usually a client-side SMTP bug.	Review the SMTP client implementation. This is rare in modern ESPs.
`501`	Bad parameters or address syntax error	The email address has an invalid format, such as a missing domain or illegal characters.	Remove the address from the list. Validate email addresses before sending.
`502`	Command not implemented	The server received a command it does not support.	Contact Blueshift Deliverability for further diagnosis.
`503`	Bad sequence of commands	The SMTP commands were sent in the wrong order.	This is an ESP-level issue. It rarely happens with Blueshift.
`504`	Command parameter not implemented	A specific parameter in the SMTP command is not supported.	This is an ESP compatibility issue and is rarely seen. Contact Blueshift Deliverability for further diagnosis.
`510`	Bad email address	The email address does not exist or is malformed.	Suppress the address from the list immediately.
`511`	Bad email address	The address is invalid. This is very similar to `510`.	Suppress the address from the list immediately.
`512`	DNS error or domain not found	The recipient domain does not exist or has no MX record.	Suppress the address. The domain is dead or invalid.
`513`	Address type incorrect	There is an address format or type mismatch.	Validate and remove invalid addresses.
`521`	Domain does not accept mail	The receiving domain explicitly rejects all incoming mail.	Suppress the address. The domain may be parked or decommissioned.
`522`	Recipient has exceeded mailbox limit	The mailbox is permanently full. Some systems treat this as a hard bounce.	Suppress the address. It is functionally inactive.
`523`	Recipient exceeds size limit	The message size exceeds the limit allowed by the recipient.	Reduce the email size or segment large attachments.
`541`	Rejected — spam or policy	The message was blocked due to spam content or a sender policy violation.	Review the content and contact Blueshift Deliverability for further diagnosis.
`550`	Mailbox unavailable or does not exist	The most common hard bounce. The recipient address does not exist.	Suppress the address from the list immediately.
`551`	User not local, try forwarding	The recipient is not local and the server suggests a forwarding address.	Update the address if a forwarding address is given. Otherwise, remove it.
`552`	Mailbox full or storage exceeded (permanent)	The mailbox is permanently over quota. Many ESPs treat this as a hard bounce.	Suppress the address. The account is likely abandoned.
`553`	Mailbox name not allowed	The recipient address violates the server naming rules.	Suppress the address. It is invalid.
`554`	Transaction failed or message rejected	A catch-all rejection for spam, blacklist, policy, or content blocks.	Contact Blueshift Deliverability for further diagnosis.
`556`	Domain does not accept mail	Similar to `521`. The domain actively rejects all mail.	Suppress the address.
`557`	Too many recipients	There are too many recipients in a single send. The batch size is exceeded.	Reduce the number of recipients per message and segment your sends.
`559`	Authentication required	The recipient server requires authentication from the sender.	Ensure SPF, DKIM, and DMARC are all configured properly. Contact Blueshift Deliverability for further diagnosis.

Enhanced status codes (RFC 3463, X.Y.Z format)

Modern mail servers return enhanced status codes in the X.Y.Z format alongside the standard three-digit SMTP code. These provide much more precise failure reasons and are increasingly used by Gmail, Microsoft, and Yahoo to communicate policy decisions.

Format breakdown

X = Class — 2 (success), 4 (transient), 5 (permanent)
Y = Subject — 1 (addressing), 2 (mailbox), 3 (mail system), 4 (network), 5 (protocol), 7 (policy)
Z = Detail code

Code	Name	What it means	Recommended action
`5.1.0`	Address rejected (generic)	A generic address rejection without a specific reason.	Review the full DSN message for details, or contact Blueshift Deliverability for further diagnosis.
`5.1.1`	Bad destination mailbox address	The address does not exist.	Hard remove the address immediately.
`5.1.2`	Bad destination mailbox address syntax	The address syntax is malformed.	Fix or remove the address.
`5.1.3`	Bad destination mailbox address syntax	Bad address syntax, similar to `5.1.2`.	Remove the address.
`5.1.6`	Destination mailbox has moved, no forwarding	The mailbox has moved with no forwarding address.	Remove the address. Check for an updated address through other channels.
`5.2.1`	Mailbox disabled or not accepting messages	The account exists but is disabled.	Treat it as a hard bounce and remove the address from the list.
`5.2.2`	Mailbox full	The mailbox storage quota is exceeded.	Treat it as a soft bounce if temporary. Suppress the address if it persists across 5 or more retries.
`5.2.3`	Message length exceeds administrative limit	The message is too large for the recipient server.	Reduce the email size and avoid heavy attachments.
`5.3.4`	Message too large for system	The system-wide size limit is exceeded.	Revisit your message to reduce its size.
`5.4.1`	No answer from host	There is no SMTP response from the destination server.	Retry. This could be a network or DNS issue.
`5.4.4`	Cannot route: invalid domain	Routing failed because the domain does not resolve.	Suppress the address. The domain is dead.
`5.5.0`	Other or undefined protocol status	An unclassified SMTP protocol error.	Review the full DSN for context, or contact Blueshift Deliverability for further diagnosis.
`5.7.1`	Delivery not authorised or policy restriction	The sending IP or domain is blocked by policy.	Check IP blacklists, SPF, DKIM, and DMARC. Warm up the IP if it is new. Contact Blueshift Deliverability for further diagnosis.
`5.7.23`	SPF validation failure (Microsoft)	Microsoft rejected the message because the sending domain's SPF record does not authorise the sending IP. The SPF lookup on the envelope-from (Return-Path) domain failed because the IP is not listed, the record has a syntax error, or too many DNS lookups caused a permerror.	Check the SPF record for the envelope-from domain using `dig TXT <domain>`. Verify that the sending IP is explicitly included. Ensure the DNS lookups in the record are 10 or fewer. A permerror occurs if this is exceeded. Use SPF flattening if needed. Microsoft is strict, so a neutral (`~all`) record may still trigger this.
`5.7.25`	DKIM validation failure (Microsoft)	Microsoft rejected the message because DKIM verification failed. The signature in the `DKIM-Signature` header did not validate against the public key published in DNS, or the signature was missing entirely.	Confirm that the DKIM selector is published correctly using `dig TXT <selector>._domainkey.<domain>`. Check that the signing domain (`d=` tag) aligns with the `From:` domain. Ensure that no intermediate server modifies the message body or headers after signing, as this breaks the hash. Verify that the key length is 1024 bits or more. 2048 bits is recommended. If you use a shared ESP infrastructure, confirm that DKIM is configured at the subdomain level.
`5.7.26`	SPF and DKIM both unauthenticated (Gmail)	Gmail rejected the message because it failed both SPF and DKIM checks. Gmail requires at least one of the two to pass and align with the `From:` domain for DMARC compliance. If neither passes, Gmail treats the message as unauthenticated and rejects it outright rather than sending it to junk.	Fix SPF. Ensure the sending IP is authorised in the SPF record of the envelope-from domain. Fix DKIM. Ensure a valid DKIM signature is present and the `d=` tag aligns with the `From:` domain. At least one must pass with alignment for DMARC to pass. Check the DMARC policy. If it is `p=reject`, failing both will always result in this error. Run a test send through mail-tester.com or Google Admin Toolbox to confirm the authentication results.
`5.7.9`	Message does not meet IPv6 sending guideline	Google: the IPv6 PTR record is missing or mismatched.	Ensure the IPv6 PTR record matches the sending hostname.

ISP-specific bounce codes

Major ISPs use standard SMTP codes, but the accompanying diagnostic message is ISP-specific and gives the real reason for the rejection. Always read the full DSN message, not just the numeric code.

Microsoft, Outlook, and Hotmail

Microsoft publishes detailed sending requirements that cover authentication, list hygiene, complaint rates, and infrastructure setup. If you encounter repeated deferrals or blocks, review these guidelines before filing a remediation request through SNDS.

ISP or provider	Code	Message	Action or notes
Microsoft / Outlook	`550 5.4.1`	Recipient address rejected: access denied	The domain or IP is blocked by Microsoft. Contact Blueshift Deliverability for further support.
Microsoft / Outlook	`550 5.7.1`	Unable to relay or policy rejection	The IP or domain is on the Microsoft block list. Contact Blueshift Deliverability for further support.
Microsoft / Outlook	`421 4.7.0`	Temporarily deferred due to IP reputation	This is a soft block. Reduce the volume. Check Microsoft SNDS signals or contact Blueshift Deliverability.

Gmail and Google Workspace

Google enforces strict requirements around SPF, DKIM, and DMARC alignment. Non-compliance results in outright rejection, not just junking. Google's Email Sender Guidelines (updated February 2024) mandate one-click unsubscribe, a spam complaint rate below 0.3%, and valid authentication for all commercial senders. For a detailed walkthrough of Google's requirements and Blueshift's recommended configuration, refer to the Gmail sender guidelines.

ISP or provider	Code	Message	Action or notes
Gmail / Google	`550 5.7.26`	This mail is unauthenticated (SPF or DKIM)	Fix the authentication. Google is strict on SPF, DKIM, and DMARC alignment. Contact Blueshift Deliverability for further support.
Gmail / Google	`421-4.7.0`	IP sending limit exceeded or rate limited	Throttle your sends and warm up gradually. Do not blast high volumes.
Gmail / Google	`550 5.2.1`	The email account is disabled	This is a hard bounce. Suppress the address immediately.

Yahoo! Mail and AOL

Yahoo follows an authentication posture similar to Gmail's and has jointly enforced bulk-sender requirements since early 2024. Senders must have valid SPF and DKIM, a published DMARC policy, and a functioning one-click unsubscribe (RFC 8058). Yahoo also enforces per-IP hourly sending limits that are not publicly documented, so throttling is the primary signal to watch. For Yahoo-specific configuration guidance used at Blueshift, refer to the Yahoo sender guidelines.

ISP or provider	Code	Message	Action or notes
Yahoo / AOL	`421 4.7.0`	Temporarily deferred; rate limit exceeded	Reduce the send rate at the campaign level. Yahoo enforces per-IP and per-hour limits.
Yahoo / AOL	`550 5.7.9`	Message not accepted for policy reasons	This is an IP or domain reputation issue, or a content policy block.
Yahoo / AOL	`554`	Message not allowed — spam or content blocked	Review the content, links, and unsubscribe mechanism.

Spamhaus, CBL, and DNSBL blocks

ISP or provider	Code	Message	Action or notes
Spamhaus / CBL	`550 5.7.1`	IP listed on Spamhaus SBL, XBL, or CBL	Investigate for a compromise or spam activity, then delist from Spamhaus. Contact Blueshift Deliverability for further support.

Bounce message interpretation

If you see this pattern	Investigate this
A high `550` or `5.1.1` rate	List hygiene issues. The addresses are old, bought, or not validated at capture.
A high `421` or `451` rate from a single ISP	Rate limiting or an IP reputation issue. Check the sending volume and IP warmup status.
`554` or `541` across multiple ISPs	Content issues, or the IP or domain is on a blacklist. Run a DNSBL check and review the email content for spam signals.
`5.7.26` from Gmail	An SPF or DKIM failure. Fix the authentication records and verify DMARC alignment.
`AUT-003` (DMARC reject)	The `From:` domain DMARC policy is `p=reject` and the authentication is failing. Escalate to Deliverability.

Recommended bounce rate thresholds

These are Blueshift's recommended alert thresholds. Exceeding them triggers a deliverability review.

Metric	Healthy	Warning zone	Critical — action required
Hard bounce rate	Below 0.5%	0.5% – 2%	Above 2% — pause the campaign and investigate the list.
Soft bounce rate	Below 2%	2% – 5%	Above 5% — check ISP throttling and IP reputation.
Spam complaint rate	Below 0.08%	0.08% – 0.3%	Above 0.3% — review immediately. Gmail will block.

Relevant resources and tools

Email bounce code reference