SMTP reply codes can look cryptic, but most of them boil down to a few categories:
- Hard bounces: permanent failures (stop sending).
- Soft bounces: temporary failures (retry with backoff).
- Rate limiting / throttling: slow down and ramp more gradually.
- Policy / reputation / authentication: fix SPF/DKIM/DMARC, content, and list hygiene.
How to interpret codes
4xx= temporary (retry)5xx= permanent (do not retry forever)- Enhanced status codes like
5.1.1(user unknown) and4.7.26(rate limiting/auth signals) provide more detail.
What to do next
- For
5.1.1/ invalid recipients: suppress immediately. - For
4.7.xthrottling: reduce concurrency, slow ramp, and focus on engaged recipients. - For
5.7.xblocks: review authentication and reputation before increasing volume.
Related docs:
Bounce Code Lookup
Search common SMTP reply codes and what to do next.
31 result(s)
550 5.1.1
User unknown / mailbox not found
Recipient address does not exist (hard bounce). Remove from list and stop retrying.
551 5.1.1
User not local / try forward-path
Recipient is not hosted on that server. Verify address or domain routing.
553 5.1.2
Domain does not exist
Recipient domain has no valid route. Double-check the domain or fix typos.
550 5.1.0
Address rejected
Mailbox/address rejected as invalid. Validate the address and suppress if persistent.
550 5.2.1
Mailbox disabled
Mailbox exists but is disabled/inactive. Suppress after repeat failures.
552 5.2.2
Mailbox full (over quota)
Recipient mailbox is over quota. Treat as transient initially; suppress if it persists.
452 4.2.2
Mailbox full (temporary)
Temporary over-quota response. Retry with backoff; stop after repeated failures.
450 4.2.0
Mailbox unavailable (temporary)
Transient mailbox issue. Retry with exponential backoff.
451 4.3.0
Temporary server error
Remote server had a transient failure. Retry with backoff.
421 4.4.1
Connection timed out
Network timeout during SMTP transaction. Retry and check destination host reachability.
421 4.4.2
Connection dropped
Remote server closed connection unexpectedly. Retry and investigate transient network issues.
554 5.4.0
No mail hosts / domain misconfigured
No MX/A records for the recipient domain. Treat as hard bounce; suppress.
421 4.7.0
Temporary deferral
Provider is deferring mail (often due to volume/reputation). Retry with pacing.
451 4.7.0
Temporary policy deferral
Temporary rejection due to policy/reputation signals. Slow down and improve authentication.
421 4.7.1
Greylisting / try again later
Recipient uses greylisting. Retry after a short delay; do not blast retries.
421 4.7.26
Gmail: Rate limited / suspicious traffic
Gmail is temporarily rate limiting. Reduce send rate, warm up, and review spam signals.
Often accompanied by text like 'This message does not pass authentication checks'.
421 4.7.28
Gmail: Temporary rate limiting
Gmail is deferring due to reputation/volume. Retry with backoff and improve list hygiene.
550 5.7.26
Gmail: Unauthenticated email
Gmail requires SPF/DKIM (and often DMARC alignment). Fix domain authentication and retry.
451 4.7.500
Outlook: Throttled / temporary rejection
Microsoft is temporarily throttling. Reduce concurrency, warm up, and retry with backoff.
550 5.7.708
Outlook: Access denied, traffic not accepted
Microsoft blocked due to reputation/policy. Ensure SPF/DKIM/DMARC, low complaints, and request delisting if needed.
421 4.7.0 [TS01]
Yahoo: Temporarily deferred (TS01)
Yahoo is deferring traffic (often reputation/volume). Slow down and retry.
421 4.7.0 [TS03]
Yahoo: Temporarily deferred (TS03)
Yahoo is temporarily deferring. Improve list quality and ramp sending gradually.
550 5.7.1
Blocked / not authorized
Rejected by policy or reputation. Verify SPF/DKIM/DMARC, review content, and check IP/domain reputation.
554 5.7.1
Message rejected (policy)
Policy-based rejection. Check authentication, spam signals, and whether the destination blocks your sending domain/IP.
550 5.7.23
SPF check failed
SPF validation failed. Fix SPF record and ensure the sending IP is authorized.
550 5.7.24
DKIM signature invalid
DKIM failed validation. Check selector, DNS record, and message signing.
550 5.7.25
Reverse DNS (PTR) missing
Some receivers require PTR/rDNS for the sending IP. Configure rDNS with your IP provider.
552 5.3.4
Message too large
Message exceeds recipient size limits. Reduce attachment size or link to downloads.
554 5.6.0
Invalid message content/format
Message is malformed (MIME/headers). Validate MIME structure, encoding, and header formatting.
554 5.7.0
Content rejected
Rejected due to spam/content filters. Reduce risky wording, ensure unsubscribe, and warm up sending.
554 5.5.0
SMTP protocol error
Protocol/command sequence error. Check SMTP client implementation and server compatibility.