What this code means
A system permanently rejected a MAIL or RCPT command because the sender or recipient address contained non-ASCII characters that were not permitted in that context. Do not retry the same unchanged attempt.
Technical meaning
The standard X.6.7 pattern means that, while receiving a MAIL or RCPT command, a system determined that a non-ASCII address was not permitted for the relevant sender or recipient. Code 5.6.7 applies this detail in class 5; the IANA registry associates the pattern with basic statuses 553 and 550.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code identifies a restriction involving a non-ASCII address, but it does not by itself prove that the mailbox does not exist or establish whether the address, system configuration, or policy caused the rejection.
- 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. Consider a new send only after a verified address correction or a change to non-ASCII address handling on the relevant path; reassess suppression before sending.
Suppression decision
Operational guidance: do not automatically add the address or domain to a suppression list based on 5.6.7 alone. First determine whether the failure occurred at MAIL or RCPT, which address it concerned, and whether the restriction was specific to the attempt, route, or policy; then apply the appropriate suppression rule.
Common causes
- The MAIL command contained a sender address with non-ASCII characters that the receiving system did not permit in that context.
- The RCPT command contained a recipient address with non-ASCII characters that the receiving system did not permit in that context.
- The system or route used for the attempt did not permit the required non-ASCII address handling because of configuration or policy.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm that the enhanced code is exactly 5.6.7 and that the basic response belongs to the 5xx class.
- Correlate the response with the intended attempt and determine whether rejection occurred at MAIL or RCPT and whether it concerned the sender or recipient address; retain the complete text and safe metadata without exposing the address.
- Inspect the address form and the configuration, capabilities, and policy of systems on the relevant path to confirm why non-ASCII characters were not permitted; do not assign the cause from the code alone.
- Stop unchanged retries; before a controlled new send, verify a material correction and reassess suppression.
Actions by owner
Sender
- Do not resend the same unchanged message; give the administrator the complete report and attempt context.
- Check the intended sender and recipient addresses, but use a different form or verified alternative address only after diagnosis; do not automatically treat the mailbox as nonexistent.
Sender administrator
- Retain the raw response, MAIL or RCPT stage, time, and route of the attempt, then identify which address and restriction involved non-ASCII characters.
- After confirming the cause, correct the verified address, configuration, or handling on the managed path, or coordinate a change with the provider; verify the correction before a new send and reassess suppression.
Provider
- For the specified attempt, inspect logs, the MAIL or RCPT stage, and the configuration and policy for non-ASCII address handling to confirm the exact rejection point.
- Remove a confirmed restriction in the managed service, or give the administrator the exact condition and safe diagnostic details; do not automatically suppress the recipient based on this code alone.
Sources and verification
The standard X.6.7 meaning, its class-5 use, and its association with basic statuses 553 and 550 were verified against the IANA registry and RFC 2034, RFC 5248, and RFC 6531 as of July 17, 2026. The diagnostic, remediation, retry, and suppression recommendations are operational guidance, not a description of any provider's practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.6.7
- 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
- rfc6531T0 source
IANA registry reference for X.6.7
Last verified:
