5.7.5: cryptographic 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.5
- 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
5.7.5 Cryptographic failure
What it means
X.7.5 covers a transport system that is supposed to validate or decrypt a message in transit and cannot, because the key it needs is unavailable or invalid. RFC 3463 ties it to no class. 5.7.5 means the message was returned; a 4.7.5 suggests the key problem may be temporary, such as a key or certificate source that did not answer. It mostly appears where gateways decrypt or verify mail on behalf of users, for example in organisations that apply S/MIME or OpenPGP at the gateway.
Common causes
- The gateway lacks the private key or certificate needed to decrypt the message.
- A certificate or key that has expired or been revoked.
- A key or certificate server that could not be reached.
How to fix it
- 01Check that you encrypted to the recipient's current certificate or key.
- 02Renew or republish your own signing certificate if it has expired.
- 03For 4.7.5, wait for the retries; the key source may come back.
- 04Recipient admins: check the gateway's keys and certificates.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.7.6Cryptographic algorithm not supported.A system that should validate or decrypt the message could not, because it does not support the algorithm the message uses.
- 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.
- 5.7.3Secure format conversion not possible.Delivery needed the message converted from one secure messaging format to another, and that conversion could not be done.
5.7.5 at other providers
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