SMTP error codes
Find the meaning of a bounce code and the next step to resolve it.
4xx usually means retry later; 5xx needs a change before resending. The enhanced code and full provider reply are more specific than the three-digit code. Search stays in your browser.
Protocol references: SMTP, enhanced status codes and SMTP authentication.
421
Service temporarily unavailable
The server cannot accept this connection right now.
Next step: Let your sending server retry. If it persists, check the receiving provider’s service status and connection limits.
450
Mailbox temporarily unavailable
Delivery was deferred, often by temporary mailbox or policy conditions.
Next step: Retry through the normal queue. Read the accompanying text for a mailbox, greylisting or policy explanation.
451
Temporary processing error
The receiving server could not process this attempt.
Next step: Keep the message queued for retry. Check DNS and provider status if many domains are affected.
452
Insufficient system storage
The receiver temporarily lacks resources.
Next step: Allow retries. If you manage the receiver, check disk space and queue capacity.
530
Authentication or encryption required
The server requires an authenticated or encrypted submission session.
Next step: Use your provider’s submission hostname, port and TLS settings. Read the response text to identify whether STARTTLS or login is required.
535
Authentication rejected
The credentials or authentication method were not accepted.
Next step: Check the full mailbox username, app password requirements and allowed authentication methods. Do not repeatedly retry a rejected password.
550
Mailbox or policy rejection
The request was permanently rejected. The accompanying enhanced code explains why.
Next step: Check the recipient spelling and the full server reply. A 550 alone does not prove the mailbox is missing.
551
User not local
The server does not host this recipient and may suggest a forwarding address.
Next step: Confirm the intended address with the recipient. Do not automatically trust an unfamiliar replacement address.
552
Storage allocation exceeded
The message exceeds a storage or message-size limit.
Next step: Check the enhanced status code. Reduce large attachments or ask the recipient to free mailbox space.
553
Mailbox name rejected
The server rejected an address or its format.
Next step: Check sender and recipient address formatting and whether the sender is allowed on this account.
554
Transaction failed
A permanent transaction failure, commonly explained by the server’s policy message.
Next step: Read the complete reply. Verify authentication and reputation before retrying the same content.
5.1.1
Destination mailbox does not exist
The local part of the recipient address was rejected as unknown.
Next step: Confirm the address with the recipient and stop retrying that address until corrected.
5.1.2
Destination domain is invalid
The destination system could not be identified or reached as addressed.
Next step: Check the domain spelling, registration and published MX records.
5.2.2
Mailbox full
The recipient mailbox has exceeded its quota.
Next step: Ask the recipient to free space or increase their quota before resending.
5.3.4
Message too large
The message exceeds the receiving system’s size limit.
Next step: Reduce attachment sizes or send a download link. Encoding makes attachments larger in transit.
4.4.1
No answer from host
A temporary connection failure prevented delivery.
Next step: Let the queue retry and check whether the recipient’s MX hosts are reachable.
4.7.1
Temporary policy deferral
The receiver temporarily deferred the message for a policy reason.
Next step: Respect the retry interval. Read the provider’s reply for throttling or authentication details.
5.7.1
Delivery not authorized
A policy or permission rule refused delivery.
Next step: Check relay permissions, SPF, DKIM and DMARC. Use any diagnostic URL in the original reply only after verifying its provider.
5.7.8
Invalid authentication credentials
The submission server rejected the login credentials.
Next step: Verify the mailbox username and app password with the provider’s documented settings.
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