X.7.27 · Security or policy
SMTP enhanced status code 5.7.27
Sender address has null MX
X.7.27 means your sender address's domain publishes a null MX, so it cannot receive bounces, and the receiver refuses mail from it.
| Registry code | X.7.27 |
|---|---|
| Forms in use | 5.7.27 |
| Registry name | Sender address has null MX |
| Subject | 7: Security or policy |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 550 |
| Defined in | [RFC 7505] |
What the registry says
This status code is returned when the associated sender address has a null MX, and the SMTP receiver is configured to reject mail from such sender (e.g., because it could not return a DSN).
IANA SMTP Enhanced Status Codes registry, X.7.27
The basic codes it travels with
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
In practice
X.7.27 is the sending-side partner of X.1.10. If your envelope domain publishes a null MX, receivers know bounces cannot be delivered, and some refuse the mail. Gmail uses 421 4.7.27 and 550 5.7.27 for a different meaning: SPF did not pass for a bulk sender. Read the text to tell which applies.
What the sender should do
Send from a domain that can receive mail, or remove the null MX from the sending domain.
Would verifying the list have prevented 5.7.27?
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.
How providers use X.7.27
Quoted from each provider’s own error reference.
- Gmail 421 4.7.27
Your email has been rate limited because SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate their email with SPF. Authentication results: SPF domain-name with IP address: ip-address = Did not pass. To set up SPF for your sending domains, visit Set up SPF. To learn more about Gmail requirements for bulk email senders, visit Email sender guidelines.
- Gmail 550 5.7.27
This message was blocked because it didn't pass SPF authentication. Gmail requires bulk email senders to authenticate their email with SPF. Authentication results: SPF with ip-address = did not pass To set up SPF for your sending domains, visit Set up SPF. To learn more about Gmail requirements for bulk email senders, visit Email sender guidelines.
Common questions
- What does 5.7.27 mean?
- X.7.27 means your sender address's domain publishes a null MX, so it cannot receive bounces, and the receiver refuses mail from it.
- What should I do about 5.7.27?
- Send from a domain that can receive mail, or remove the null MX from the sending domain.
- Would an email verifier have caught 5.7.27?
- 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.
Full guides for this code
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)
- Google Workspace Admin Help: Gmail SMTP errors and 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