4.x.x — Temporary failure codes
Class 4 enhanced status codes mark a transient failure. Delivery did not complete, but a controlled retry can be appropriate after you check timing, limits, routing and the full response.
A leading 4 is a soft failure: the condition may clear on its own. Retry with backoff rather than suppressing the recipient, and confirm the exact reason from the complete SMTP response before changing send behavior.
Codes in this category
This inventory contains only reviewed codes assigned to this subject. Open a code to see its evidence, diagnostic steps and owner-specific actions.
- 4.0.0Other undefined temporary statusCode 4.0.0 is a general temporary failure: only class 4 is known, with no more specific status. The message may be retried in a controlled way.CategoryOther and undefinedRecommended actionControlled retry
- 4.1.1Temporary destination mailbox address problemThe receiving system did not find the specified mailbox during this attempt. Because the code begins with 4, this is a temporary failure: delivery may be retried in a controlled way, but the code alone does not yet establish that the address is permanently invalid.CategoryAddressingRecommended actionControlled retry
- 4.1.8Temporary sender system address problemThis attempt reported a problem with the system identified by the sender address. Because the code begins with 4, it is a temporary failure: delivery may be retried in a controlled way, but the sender address and system configuration should be checked first.CategoryAddressingRecommended actionControlled retry
- 4.2.4Temporary mailing-list expansion problemThe receiving system recognized the destination as a mailing list but could not expand it to recipients during this attempt. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryRecipient mailboxRecommended actionControlled retry
- 4.3.0Other or undefined temporary mail-system statusThe destination system exists and normally accepts mail, but a general problem in that system prevented delivery during this attempt. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
- 4.3.1Temporarily full mail systemThe mail system exceeded its available storage during this attempt. This is a system-wide resource problem, so an individual recipient may be unable to free space on their own. Because the code begins with 4, delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
- 4.3.2System temporarily not accepting network messagesThe host on which the recipient's mailbox resides is not currently accepting messages. Because the code begins with 4, this is a temporary failure and delivery may be retried in a controlled way.CategoryMail systemRecommended actionControlled retry
- 4.4.1No answer from remote hostAn outbound connection attempt received no answer from the remote system. The standard indicates that the system may have been busy or unable to accept a connection. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
- 4.4.2Bad connectionAn outbound connection was established, but the message transaction could not be completed over it. The standard points to a timeout or inadequate connection quality. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
- 4.4.3Directory server failureThe network system could not forward the message because a directory server was unavailable. Inability to connect to an Internet DNS server is one standards example, but the code alone does not prove that the problem involved DNS. The code belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
- 4.4.5Mail system congestionA mail system could not deliver the message because it was congested. The code does not identify which system or party is responsible for that condition. It belongs to the temporary class, so delivery may be retried in a controlled way.CategoryNetwork, DNS and routingRecommended actionControlled retry
- 4.5.0Other or undefined temporary protocol statusA problem occurred with the protocol needed to pass the message to the next system, but no other detail code described it adequately. The leading digit 4 makes this a temporary result, so delivery may be retried in a controlled way.CategoryDelivery protocolRecommended actionControlled retry
- 4.5.1Temporarily invalid protocol commandA mail system rejected a protocol command because it was issued out of sequence or was unsupported. The leading digit 4 classifies this concrete result as temporary, so delivery may be retried in a controlled way after the command context is checked.CategoryDelivery protocolRecommended actionControlled retry
- 4.5.3Too many recipientsOne message specified more recipients than the protocol could handle in that attempt. The code belongs to the temporary class; normally, the remaining recipients should be separated and delivery to them retried in a controlled way.CategoryDelivery protocolRecommended actionControlled retry
- 4.5.4Temporarily invalid command argumentsA mail system recognized the command but rejected its arguments as invalid. The leading digit 4 classifies this concrete result as temporary, so delivery may be retried in a controlled way after the arguments are checked and, when necessary, corrected.CategoryDelivery protocolRecommended actionControlled retry
- 4.7.0Other or undefined temporary security statusThe message was returned because of a security-related problem, but the response does not describe it more precisely. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.CategorySecurity, authentication and policyRecommended actionControlled retry
- 4.7.1Delivery not authorized, message temporarily refusedThe system refused the current attempt because the sender was not authorized to send to the destination. The decision may result from per-host or per-recipient filtering. The leading digit 4 denotes a temporary failure, so delivery may be retried in a controlled way.CategorySecurity, authentication and policyRecommended actionControlled retry
- 4.7.12Authentication mechanism transition requiredThe server replied to the AUTH command that the user must first transition to the selected authentication mechanism. The leading digit 4 denotes a temporary failure, so a controlled attempt may be made after the required transition is completed safely.CategorySecurity, authentication and policyRecommended actionControlled retry
- 4.7.15Priority level is too lowThe receiving SMTP server did not accept the message because the specified priority level was below the lowest level the server currently accepts. The leading digit 4 denotes a temporary failure, so a controlled retry is appropriate.CategorySecurity, authentication and policyRecommended actionControlled retry
- 4.7.16Message too big for the specified priorityThe server temporarily rejected the message because it is too big for the specified priority. A controlled retry may be made after the condition clears.CategorySecurity, authentication and policyRecommended actionControlled retry
- 4.7.24SPF validation errorAn error occurred while SPF was evaluated for the arriving message, so the current attempt ended in a temporary failure. A controlled retry is appropriate, not unlimited retries.CategorySecurity, authentication and policyRecommended actionControlled retry
Wojtek Blazalek
Email deliverability expert
Stuck on this error code? I help teams clear the root cause of rejections and fix authentication and reputation — so email lands in the inbox.
Hands-on deliverability work for teams that send at scale.