X.7.15 · Security or policy
SMTP enhanced status code 4.7.15
Priority Level is too low
X.7.15 means the priority level you requested, under RFC 6710, is too low for the receiving server at the moment. It may be temporary.
| Registry code | X.7.15 |
|---|---|
| Forms in use | 4.7.15, 5.7.15 |
| Registry name | Priority Level is too low |
| 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 | 450, 550 (other 4XX or 5XX codes are allowed) |
| Defined in | [RFC 6710] |
What the registry says
The specified priority level is below the lowest priority acceptable for the receiving SMTP server. This condition might be temporary, for example the server is operating in a mode where only higher priority messages are accepted for transfer and delivery, while lower priority messages are rejected.
IANA SMTP Enhanced Status Codes registry, X.7.15
The basic codes it travels with
- 450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
In practice
X.7.15 belongs to the SMTP priority extension in RFC 6710. It means the server is accepting only higher-priority mail for now. Very few public servers use the extension, so you are unlikely to see it unless you set priorities deliberately.
What the sender should do
Resend without a priority request, or at a higher priority if you are allowed one.
Would verifying the list have prevented 4.7.15?
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 4.7.15 mean?
- X.7.15 means the priority level you requested, under RFC 6710, is too low for the receiving server at the moment. It may be temporary.
- What is the difference between 4.7.15 and 5.7.15?
- They share the subject and detail digits, so they report the same condition, "Priority Level is too low". 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 4.7.15?
- Resend without a priority request, or at a higher priority if you are allowed one.
- Would an email verifier have caught 4.7.15?
- 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)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply 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