X.1.9 · Addressing
SMTP enhanced status code 5.1.9
Message relayed to non-compliant mailer
X.1.9 says the address was valid, but the message was handed on to a system that does not speak the delivery status protocol, so no further reports will come back. It is informational rather than a failure in most logs.
| Registry code | X.1.9 |
|---|---|
| Forms in use | 5.1.9, 4.1.9 |
| Registry name | Message relayed to non-compliant mailer |
| Subject | 1: Addressing |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | Not given |
| Defined in | [RFC 3886] |
What the registry says
The mailbox address specified was valid, but the message has been relayed to a system that does not speak this protocol; no further information can be provided.
IANA SMTP Enhanced Status Codes registry, X.1.9
In practice
X.1.9 comes from older gateway setups where mail passes into a system that cannot produce delivery status notifications, such as some legacy or proprietary systems. It tells you only that the message left the standard SMTP world. You may see it in DSNs with Action: relayed rather than failed. It does not indicate a problem with the address and is not a bounce in the list-hygiene sense.
What the sender should do
No action is normally needed. If delivery matters, confirm receipt with the recipient, because no later status will arrive.
Would verifying the list have prevented 5.1.9?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does 5.1.9 mean?
- X.1.9 says the address was valid, but the message was handed on to a system that does not speak the delivery status protocol, so no further reports will come back. It is informational rather than a failure in most logs.
- What is the difference between 5.1.9 and 4.1.9?
- They share the subject and detail digits, so they report the same condition, "Message relayed to non-compliant mailer". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.1.9?
- No action is normally needed. If delivery matters, confirm receipt with the recipient, because no later status will arrive.
- Would an email verifier have caught 5.1.9?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05