5.7.3: secure format conversion not possible
Delivery needed the message converted from one secure messaging format to another, and that conversion could not be done.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.3
- 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
5.7.3 Security conversion required but not possible
What it means
X.7.3 concerns protected messages, for example signed or encrypted with one standard and headed for a system that needs another. When delivery requires converting between secure messaging protocols and that is not possible, the server uses this code. RFC 3463 calls it useful only as a permanent error, so 5.7.3 is the form you meet and the message is returned. It is rare, and mostly found at gateways between different secure messaging environments. Sending the message in the format the destination supports, or without that protection, gets it through.
Common causes
- An encrypted or signed message sent towards a system that requires a different secure format.
- A gateway without the keys or software to translate between the two formats.
How to fix it
- 01Ask the recipient which secure format their system supports (for example S/MIME or OpenPGP) and use that.
- 02If protection is not needed for this message, send it without signing or encryption.
- 03Recipient admins: check the gateway's support for the incoming format.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.7.4Security feature not supported.The message or session asked for a security feature, such as a form of secure authentication, that the delivery protocol here cannot support.
- 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.
- 5.7.6Cryptographic algorithm not supported.A system that should validate or decrypt the message could not, because it does not support the algorithm the message uses.
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