X.1.8 · Addressing
SMTP enhanced status code 5.1.8
Bad sender's system address
X.1.8 means the domain in your sender address does not exist or cannot receive replies, so the receiving server will not accept mail that could not be bounced back. It often follows a typo in the sending domain or a missing MX or A record for a subdomain.
| Registry code | X.1.8 |
|---|---|
| Forms in use | 5.1.8, 4.1.8 |
| Registry name | Bad sender's system address |
| 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 | 451, 501 |
| Defined in | [RFC 3463] |
What the registry says
The sender's system specified in the address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the "@" is invalid for mail.
IANA SMTP Enhanced Status Codes registry, X.1.8
The basic codes it travels with
- 451 Requested action aborted: local error in processing
- 501 Syntax error in parameters or arguments
In practice
Receivers check that your sender domain can accept mail, because bounces and replies need somewhere to go. X.1.8 appears when the envelope domain has no MX or A record, for example a new subdomain used as a return-path that was never given DNS records, or a typo in the sending domain. Yahoo uses an SOA lookup for this check, so subdomains used in MAIL FROM need DNS records of their own.
What the sender should do
Make sure the domain in MAIL FROM resolves and can receive mail, including any bounce or return-path subdomain your sending service uses.
Would verifying the list have prevented 5.1.8?
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.
How providers use X.1.8
Quoted from each provider’s own error reference.
- Microsoft 365 5.1.8
Access denied, bad outbound sender
Common questions
- What does 5.1.8 mean?
- X.1.8 means the domain in your sender address does not exist or cannot receive replies, so the receiving server will not accept mail that could not be bounced back. It often follows a typo in the sending domain or a missing MX or A record for a subdomain.
- What is the difference between 5.1.8 and 4.1.8?
- They share the subject and detail digits, so they report the same condition, "Bad sender's system address". 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 5.1.8?
- Make sure the domain in MAIL FROM resolves and can receive mail, including any bounce or return-path subdomain your sending service uses.
- Would an email verifier have caught 5.1.8?
- 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.
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)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (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