X.7.6 · Security or policy
SMTP enhanced status code 5.7.6
Cryptographic algorithm not supported
X.7.6 means the cryptographic algorithm used is not supported by the system that had to validate or decrypt the message.
| Registry code | X.7.6 |
|---|---|
| Forms in use | 5.7.6, 4.7.6 |
| Registry name | Cryptographic algorithm not supported |
| Subject | 7: Security or policy |
| 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
A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.
IANA SMTP Enhanced Status Codes registry, X.7.6
In practice
X.7.6 means the algorithm used for signing or encryption is not supported by the system that had to process it. Old or uncommon algorithms are the usual cause. Switching to a widely supported algorithm fixes it.
What the sender should do
Use a more widely supported algorithm.
Would verifying the list have prevented 5.7.6?
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 5.7.6 mean?
- X.7.6 means the cryptographic algorithm used is not supported by the system that had to validate or decrypt the message.
- What is the difference between 5.7.6 and 4.7.6?
- They share the subject and detail digits, so they report the same condition, "Cryptographic algorithm not supported". 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 5.7.6?
- Use a more widely supported algorithm.
- Would an email verifier have caught 5.7.6?
- 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