TL;DR
Permanent generic address status: identify the affected envelope address before acting.
What this code means
X.1.0 is IANA’s other-address-status detail, with no IANA class confirmation for this pattern in the catalog. Exact corpus evidence for 5.1.0 spans unauthorized sender identities at Orange and Rackspace and a recipient-address rejection through a Cisco-hosted gateway.
Provider examples
smtp;501 5.1.0 Emetteur invalide. Invalid Sender. OFR004_405 [405]550 5.1.0 Sender is not allowed to send from example.com550 #5.1.0 Address rejected.Technical meaning
The same concrete code can describe a sender-envelope or recipient-envelope condition. The full response and the SMTP command it answers, not the digits alone, identify the owner.
Delivery status
Class 5 makes the unchanged attempt permanent. It does not make every 5.1.0 response a recipient hard bounce.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Stop retries of the unchanged send. Repair the sender identity or recipient address identified by the response before a new attempt.
Suppression decision
Do not automatically suppress a recipient or domain from 5.1.0. If the response concerns MAIL FROM, route it to sender-identity remediation; use address-specific hygiene only for a confirmed RCPT TO condition.
Common causes
- The provider uses generic address status for an unauthorized sender identity or a rejected recipient address.
Diagnostic steps
- Keep the raw reply and determine whether it answered MAIL FROM or RCPT TO before selecting an owner.
Actions by owner
Sender
- Correct a confirmed recipient address only from a trusted source; do not guess a replacement.
Sender administrator
- For a sender-identity response, inspect envelope-sender authorization and the exact provider text before sending again.
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.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- Rackspace Email SMTP response codes — Rackspace postmaster reference for SMTP response and reject codes.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
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