5.1.8SMTP 5.1.8: The sender system address is invalid

X.1.8 means the system identified by the sender address does not exist or cannot accept return mail. In an Internet address, this concerns the part to the right of @ in the sender address, not the local part and not the recipient. 5.1.8 is the permanent, class 5 form of that status.

The address this code concerns is the envelope sender given in MAIL FROM, the reverse path that a receiving system records as Return-Path on delivery and the address any bounce would be sent back to. The sending system sets that address, and it can differ from the From header shown to the reader, so a correct-looking From header does not clear the condition and does not change what has to be checked. The response reports that the system named to the right of @ in the envelope sender does not exist or cannot receive return mail; it reports nothing about the recipient mailbox or about the message itself.

Class 5 is permanent for the attempt as sent. RFC 2034 requires the enhanced status class to agree with the SMTP reply class, so 5.1.8 arrives on a 5xx reply and never on a temporary one. RFC 5248 records the associated basic status code as non-exclusive, so the three-digit reply carrying the code varies between systems, and the enhanced code together with the full response text is what identifies the condition. The status describes the sender system, so it does not support recipient suppression.

Class

Permanent failure

Retry

Do not retry unchanged

Suppression

Check the full context

Do not retry the message as sent; the same envelope sender produces the same permanent result. A new attempt is authorized only after the envelope sender domain resolves and accepts return mail, and after that correction has been confirmed outside the sending queue. A send from a corrected envelope sender is a new route rather than a retry of the rejected attempt, and it counts as the first controlled test of the correction.

5.1.8 evaluates the sender system, so it carries no verdict on the recipient address and is not grounds for suppressing that recipient. Record the event against the sender domain and its return path, and leave suppression to codes that report the destination address. Where a recipient is already suppressed on other evidence, that decision stands on its own record and is not confirmed by this code.

• The domain in the envelope sender does not exist in DNS, or exists as a name with no mail configuration at all, so the receiving system finds no sender system that could take return mail.
• The domain resolves, but there is no working return route to it: no reachable mail exchanger, or a published host that no system answers for, so mail addressed back to the sender has nowhere to go.
• The domain and its routing exist, but the bounce configuration names something else: a return-path subdomain or bounce domain that was never published or has since lapsed, or an envelope sender rewritten by an intermediate system into an address the sending organization does not host.

1. Read the complete SMTP response as the receiving system returned it, and confirm that the enhanced code is 5.1.8 and which 5xx reply carried it; the response text, not the digits alone, states which address was rejected.
2. Take the envelope sender from MAIL FROM in the transaction log for that message and compare it with the From header; where the two differ, the envelope sender is the address the code concerns, and it is the address carried into the rest of the check.
3. Check DNS and return routing for that envelope sender domain: whether the domain resolves, whether it publishes a reachable route for mail addressed back to it, and whether the return-path or bounce host named in the sending configuration is the one actually published.
4. Make one controlled attempt only after a corrected envelope sender domain resolves and accepts return mail: a single message to a destination known to be reachable, with the full response retained, so the result attaches to the correction and not to a batch.