Standard SMTP codes · permanent error
5.7.6: cryptographic algorithm not supported
A system that should validate or decrypt the message could not, because it does not support the algorithm the message uses.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.6
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.7.6 Cryptographic algorithm not supported
What it means
X.7.6 is close to X.7.5, except that the key is fine: the gateway or server does not implement the algorithm used to sign or encrypt the message. RFC 3463 ties it to no class. The same algorithm will meet the same system on every attempt, so 5.7.6 is the practical form and the message is returned. The fix is to sign or encrypt with an algorithm that both sides support, which usually means a common, current one.
Common causes
- A message encrypted or signed with an algorithm the gateway does not implement.
- An old gateway without support for newer algorithms, or a sender using an outdated one.
How to fix it
- 01Ask the recipient which algorithms their system supports.
- 02Change your client's signing or encryption settings to a widely supported algorithm.
- 03Recipient admins: update the gateway's cryptographic software.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.7.5Cryptographic failure.A system that was authorised to check or decrypt the message could not do it, because a key or other required information was missing or invalid.
- 5.7.7Message integrity check failed.A system authorised to validate the message found it corrupted or altered, so the check on its integrity failed.
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