What this code means
The receiving system permanently rejected the message because the associated sender address has a null MX and the receiver rejects mail from such senders—for example, when it could not return a DSN. Do not retry the same attempt unchanged.
Technical meaning
The standard X.7.27 pattern means that the associated sender address has a null MX and the SMTP receiver is configured to reject mail from such a sender. Code 5.7.27 applies this 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 and verification
The standard meaning of X.7.27, its relationship to a null MX and SMTP-receiver policy, and its class-5 application were verified against the IANA registry and RFC 2034, RFC 5248, and RFC 7505 as of July 17, 2026. Diagnostic, remediation, retry, and suppression recommendations are separate operational guidance; this record contains no provider-specific practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.7.27
- rfc5248T0 source
Section 2.1: registry fields and non-exclusive Associated Basic Status Code
- rfc2034T0 source
Section 4: enhanced status class agrees with SMTP reply class
- rfc7505T0 source
IANA registry reference for X.7.27
Last verified:
