TL;DR
Permanent destination rejection: fix the exact address; do not infer a domain-wide failure.
What this code means
RFC 3463 describes X.1.2 as a bad destination system address and permanent-only detail, but this catalog has no IANA class confirmation for the pattern. Exact Gmail 553 evidence instead rejects recipient-address syntax, a narrower provider-specific reading.
Provider examples
smtp;553 5.1.2 The recipient address <example@example.com> is not a valid RFC-5321 address. - gsmtp553 5.1.2 The recipient address <example@example.com> is not a valid RFC-5321 address. - gsmtpTechnical meaning
The evidence supports a malformed destination address at Gmail, not a general proof that every 5.1.2 means a nonexistent domain. Read the full response.
Delivery status
Class 5 is permanent for the unchanged attempt. It does not make unrelated addresses at the domain invalid.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Do not retry the same address. Retry only after a verified correction or a confirmed change at the identified destination system.
Suppression decision
Apply any hygiene decision to the confirmed address only. Do not automatically suppress the whole domain from a single 5.1.2 response.
Common causes
- The provider rejects the supplied recipient address as invalid under its syntax check.
Diagnostic steps
- Confirm the complete reply and recipient address, then distinguish provider-specific format text from a destination-system condition.
Actions by owner
Sender
- Stop sends to the unchanged address and obtain a correction through a trusted channel.
Sender administrator
- Inspect source-data validation and retain the response before applying address-specific hygiene.
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.
- RFC 5321 — Simple Mail Transfer Protocol — The core SMTP standard, including valid mailbox address syntax.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
Last verified:
