Enhanced SMTP status code · RFC 3463 · IANA
5.7.3 Security conversion not possible
Delivery required converting from one secure messaging protocol to another, and that was not possible.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code is only useful as a permanent failure.
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
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Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026