Yahoo/AOL 451
Temporary server failure
Temporary server failure means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.
Decision
A retry can work, especially after a short wait.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- Any SMTP stage
- Endpoint
- Inbound MX
Resources temporarily unavailableStart here
- 1
Read the full SMTP diagnostic text, not only the numeric reply code.
- 2
Identify the provider, SMTP stage, and any provider token before deciding whether to retry or suppress.
Confirm the cause
- Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
- Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
- Test with one recipient before retrying bulk traffic.
App handling
- Queue a retry with backoff and keep the original SMTP reply attached to the attempt.
- Slow or pause provider-specific sending if the same deferral repeats across many recipients.
- Escalate to reputation, authentication, or volume review when retries keep failing.
Avoid
- Do not suppress the recipient from a temporary response.
- Do not keep the same retry pace if the deferral repeats across many recipients.
- Do not ignore sender reputation or authentication if this keeps happening.
Common causes
- The provider-specific diagnostic text matches this class of failure.
- The same SMTP code may have another meaning when the provider, stage, or endpoint differs.
- Sender setup, recipient status, and provider policy should all be checked before taking irreversible action.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp