What this code means
The message was permanently rejected because the recipient mailbox address has invalid syntax. Retrying with the same address will not remove the error.
Technical meaning
The X.1.3 pattern means that the destination mailbox address is syntactically invalid; the error can apply to any field in that address. The concrete 5.1.3 code combines this meaning with permanent class 5, for which the standard defines this detail.
Delivery status
The leading digit 5 denotes a permanent failure. The current delivery request should not be retried unchanged, although correcting the address from reliable information may allow a new send.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries with the unchanged address. Send again only after obtaining and applying a reliable address correction, treating it as a new attempt with corrected data.
Suppression decision
Operational guidance: evaluate suppression in the context of the exact address and the source of the invalid value. Do not blanket-suppress the recipient, domain, or broader traffic from a single 5.1.3 result; a confirmed unusable address may be blocked under local policy.
Common causes
- The source data contains a destination mailbox address with invalid syntax.
- The message-construction process produced a syntactically invalid destination-address field.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 5.1.3 and that the basic reply is in the 5xx class.
- Compare the complete destination address with a trusted source value and check the syntax of each field; do not guess the correct address.
- Trace the address from input data to the completed message to determine whether the invalid syntax came from the stored value or the message-construction process.
Actions by owner
Sender
- Verify the address against a value supplied or confirmed by the recipient, and change it only when you have a reliable correction.
- Do not resend to the same address; if it cannot be confirmed, request a correct address through another available channel.
Sender administrator
- Classify 5.1.3 as a permanent error and stop retries of the unchanged send while retaining the raw response for diagnosis.
- Review input validation and address-field construction, and limit any suppression decision to the confirmed address and local policy instead of applying a blanket rule.
Sources and verification
The X.1.3 meaning, its permanent use, and class-interpretation rules are standards facts from the IANA registry and RFC 2034, RFC 3463, and RFC 5248, verified on July 17, 2026. Operational retry, suppression, diagnostic, and owner-action guidance is separated from those facts; no provider-practice facts have been accepted.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.1.3
- 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
- rfc3463T0 source
IANA registry reference for X.1.3
Last verified:
