TL;DR
Permanent rejection: the sender domain has a null MX and the receiver rejects such mail. Use a sendable From domain or adjust recipient policy before retrying. Null MX senders often fail deliverability and DSN expectations. Do not retry unchanged.
What this code means
Null MX on the domain of the SMTP transaction's sender address, combined with a receiving SMTP server configured to reject such senders, is registry pattern X.7.27 returned as code 5.7.27. The leading digit denotes a permanent failure in the enhanced status register. The subcode couples sender DNS publication with receiver policy, not recipient mailbox validity. The code alone does not prove whether the null MX is deliberate, whether the From domain should change, or whether the rejection policy on the receiving side should be adjusted.
Provider examples
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. - gsmtpTechnical meaning
Associated sender addresses that publish a null MX, when the SMTP receiver is configured to reject mail from such senders, match X.7.27. Code 5.7.27 applies that meaning in the permanent-failure class.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code alone does not establish whether the null MX is intentional or whether the sender address, DNS, or receiving policy needs correction.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries of the same unchanged attempt. Make a new, controlled attempt only after a confirmed change to the sender address, its MX, or the receiving policy, and after checking suppression again.
Suppression decision
Operational guidance: do not automatically suppress the recipient address or domain based on 5.7.27 alone. The code concerns the sender address and receiving-system policy, so suppression requires the complete context and a confirmed cause.
Common causes
- The sender address associated with the SMTP transaction belongs to a domain with a null MX, and the receiving system is configured to reject such senders.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report, confirm the exact 5.7.27 code, and retain the complete response text.
- Correlate the rejection with the correct attempt and identify the sender address and domain to which the code applied.
- Inspect the domain's current MX data and confirm whether the null MX is intentional; do not infer a particular provider's practice from the code alone.
- Inspect the receiving system's logs and policy. Stop unchanged retries; after a confirmed correction, check suppression again and make one controlled attempt.
Actions by owner
Sender
- Do not manually retry the same unchanged message; give the sender administrator the complete response, attempt time, and sender address used.
Sender administrator
- Correlate the response with the sender address used, inspect its domain's MX data, and establish whether the null MX is intentional or a configuration error.
- If the null MX is erroneous, correct DNS; if it is intentional, use an appropriate sender address that can receive a DSN. Only then check suppression and make one controlled attempt.
Recipient administrator
- If you manage the system that returned the code, inspect its logs and policy for rejecting senders with a null MX; correct a confirmed configuration error or give the sender the precise rejection context.
Provider
- If you operate managed DNS, sending, or receiving infrastructure, inspect the relevant configuration and logs, correct a confirmed problem in that layer, or give the administrators the evidence needed for remediation.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 7505 — A "Null MX" No Service Resource Record — Defines Null MX for domains that intentionally accept no mail.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
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