X.4.4 · Network and routing
SMTP enhanced status code 5.4.4
Unable to route
X.4.4 means the mail system could not work out where to send the message, because routing information was missing, for example a domain with no MX or A record. It can be temporary or permanent.
| Registry code | X.4.4 |
|---|---|
| Forms in use | 5.4.4, 4.4.4 |
| Registry name | Unable to route |
| Subject | 4: Network and routing |
| 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 3463] |
What the registry says
The mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server. This is useful for both permanent and persistent transient errors. A DNS lookup returning only an SOA (Start of Administration) record for a domain name is one example of the unable to route error.
IANA SMTP Enhanced Status Codes registry, X.4.4
In practice
X.4.4 is the routing failure: the domain exists but DNS gives no usable mail route, such as no MX and no A record, or only an SOA record. Some servers report it as temporary in case DNS is being changed, others as permanent. If it persists, every address on that domain is undeliverable, and verification flags these domains during the DNS step.
What the sender should do
Check the recipient domain's DNS. If it has no mail records, remove its addresses; a verifier flags these during the DNS step.
Would verifying the list have prevented 5.4.4?
Yes, verification catches this
A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Common questions
- What does 5.4.4 mean?
- X.4.4 means the mail system could not work out where to send the message, because routing information was missing, for example a domain with no MX or A record. It can be temporary or permanent.
- What is the difference between 5.4.4 and 4.4.4?
- They share the subject and detail digits, so they report the same condition, "Unable to route". 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 5.4.4?
- Check the recipient domain's DNS. If it has no mail records, remove its addresses; a verifier flags these during the DNS step.
- Would an email verifier have caught 5.4.4?
- Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Full guides for this code
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