TL;DR
Invalid address syntax: correct the exact recipient before resending.
What this code means
X.1.3 denotes syntactic invalidity of the destination mailbox address and may apply to any field in that address. Code 5.1.3 combines it with permanent class 5.
Provider examples
553 5.1.3 The recipient address address is not a valid RFC 5321 address. For more information, go to About SMTP error messages and review RFC 5321 specifications. - gsmtp501 5.1.3 Invalid address [outlook.com]Technical meaning
This is a syntax condition on the destination mailbox address, not a claim that every address at the domain is unusable.
Delivery status
The unchanged request is permanently rejected; correcting the input is the relevant change.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Do not retry unchanged. Resend only after a trusted correction to the recipient address.
Suppression decision
Use an address-specific data-quality action if local policy requires it; do not suppress the domain from this result alone.
Common causes
- The destination mailbox address contains invalid syntax.
Diagnostic steps
- Retain the raw response and compare the submitted address with its authoritative source before editing it.
Actions by owner
Sender
- Stop using the unchanged address and request a verified correction.
Sender administrator
- Inspect address validation at the source and record the result against this recipient only.
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 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
Last verified:
