X.7.21 · Security or policy
SMTP enhanced status code 5.7.21
No acceptable DKIM signature found
X.7.21 means the message had passing DKIM signatures, but none from a domain the receiver accepts.
| Registry code | X.7.21 |
|---|---|
| Forms in use | 5.7.21 |
| Registry name | No acceptable DKIM signature found |
| Subject | 7: Security or policy |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 550 |
| Defined in | [RFC 7372]; [RFC 6376] |
What the registry says
This status code is returned when a message contains one or more passing DKIM signatures, but none are acceptable. (This violates the advice of [Section 6.1 of RFC 6376].)
IANA SMTP Enhanced Status Codes registry, X.7.21
The basic codes it travels with
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
In practice
X.7.21 means a DKIM signature passed, but its signing domain is not one the receiver accepts, for example only the sending platform's shared domain. Signing with your own domain, by publishing the DKIM record your platform gives you, usually fixes it. Receivers that require a signature from the author's domain, or from a list of trusted domains, return this code when only a third-party signature is present. Check the d= value in the signature.
What the sender should do
Sign with your own domain rather than only the sending service's default domain.
Would verifying the list have prevented 5.7.21?
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 5.7.21 mean?
- X.7.21 means the message had passing DKIM signatures, but none from a domain the receiver accepts.
- What should I do about 5.7.21?
- Sign with your own domain rather than only the sending service's default domain.
- Would an email verifier have caught 5.7.21?
- 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)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply 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