SMTP Addressing Status Codes
Use X.1 status codes when an SMTP response concerns recipient addressing, destination selection, or an address-derived routing condition. Keep the full response: the leading class determines whether the reported result was successful, temporary, or permanent.
The addressing subject groups conditions about the destination or sender address rather than every possible delivery failure. An X.1 code can confirm an address action, report a temporary recipient-side condition, or identify a permanent addressing problem; it is not, by itself, a suppression instruction.
Choose the next check
- Keep the exact address responsePreserve the full SMTP reply, enhanced code, and recipient context before changing address data or recipient status.
- Check recipient syntax and existenceUse the detailed code to distinguish a malformed recipient address from a mailbox that the destination reports as unavailable.
- Check mailbox availability and recipient-address validityA temporary unavailable mailbox and a recipient-address validity failure need different actions; use the exact code before changing recipient data.
Common mistakes
- Treating every X.1 result as a permanent invalid-recipient signal.
- Changing or suppressing an address without retaining the complete response and recipient context.
- Assuming an address-related failure is necessarily a mailbox-existence failure.
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.
- 2.1.5Destination address is validThe specified recipient mailbox address was found to be valid. This is a success confirmation, not a message rejection.Delivery status2xx · SuccessRecommended actionCheck the full context
- 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.Delivery status4xx · Temporary failureRecommended 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.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 5.1.0Generic address-status rejection, evidenced for both sender and recipient addressesThe receiving system reports a permanent failure under registry pattern X.1.0, IANA's catch-all "other address status" detail. No class is registry-confirmed for this pattern; the exact production evidence in this catalog covers two distinct situations — Orange and Rackspace rejecting an unauthorized or misaligned envelope sender, and a Cisco-routed non-delivery report rejecting the envelope recipient address — so the same code needs the full response text to know which address it names.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.1.1Recipient mailbox does not existThe specified recipient mailbox does not exist. This is a permanent failure, so do not retry delivery to the same unchanged address.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.1.2Recipient address rejected as an invalid destinationThe receiving system reports that the recipient address itself failed its address-validity check and was rejected at the RCPT TO stage. The code belongs to the permanent class: correct or remove the address before sending again, and do not retry the same unchanged value.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.1.3Invalid recipient address syntaxThe message was permanently rejected because the recipient mailbox address has invalid syntax. Retrying with the same address will not remove the error.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.1.8Invalid sender system addressThe system identified by the sender address does not exist or cannot accept return mail. Code 5.1.8 is a permanent failure, so do not retry the unchanged send.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.1.10Recipient address points to a null-MX domainThe recipient address is associated with a domain marked by a null MX as not accepting mail. This is a permanent failure, so do not retry delivery to the same unchanged address.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Related resources
Guide
- List Management
Classify address failures as hard vs soft before suppress.
- Deliverability
Hard address bounces damage reputation if retried.
Incidents
- Why is our bounce rate high?
Separate permanent address fails from temporary ones.
Wojtek Blazalek
Email deliverability expert
Stuck on this error code? I help teams identify rejection causes and fix authentication and reputation, so email reaches the inbox.
Hands-on deliverability work for teams that send at scale.