X.7.3 · Security or policy
SMTP enhanced status code 5.7.3
Security conversion required but not possible
X.7.3 means delivery needed a conversion between secure messaging formats that could not be done.
| Registry code | X.7.3 |
|---|---|
| Forms in use | 5.7.3, 4.7.3 |
| Registry name | Security conversion required but not possible |
| 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 conversion from one secure messaging protocol to another was required for delivery and such conversion was not possible. This is useful only as a permanent error.
IANA SMTP Enhanced Status Codes registry, X.7.3
In practice
X.7.3 appears where secure messaging systems, such as S/MIME or PGP gateways, have to translate between formats and cannot. It is rare outside regulated industries. Sending the message without the secure format, or agreeing a common format with the recipient, solves it.
What the sender should do
Send the message without the secure-messaging format the recipient cannot handle, or agree on a common one.
Would verifying the list have prevented 5.7.3?
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 5.7.3 mean?
- X.7.3 means delivery needed a conversion between secure messaging formats that could not be done.
- What is the difference between 5.7.3 and 4.7.3?
- They share the subject and detail digits, so they report the same condition, "Security conversion required but not possible". 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 5.7.3?
- Send the message without the secure-messaging format the recipient cannot handle, or agree on a common one.
- Would an email verifier have caught 5.7.3?
- 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