TL;DR
Mail-system storage is temporarily full. Retry with backoff and do not suppress the recipient from 4.3.1 alone.
What this code means
X.3.1 means mail-system storage has been exceeded; 4.3.1 is its transient result.
Provider examples
452 4.3.1 Insufficient system resources (TSTE) [outlook.com]Technical meaning
The storage shortage is system-wide or administrative and need not be solvable by one recipient deleting mail.
Delivery status
Class 4 means capacity may be restored; it is not a permanent rejection or address defect.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Use bounded idempotent backoff retries and stop on success, permanent result, or exhaustion.
Suppression decision
Do not suppress from 4.3.1 alone; require independent permanent evidence or policy.
Common causes
- Mail-system storage was exceeded at the time of the attempt.
Diagnostic steps
- Retain the raw response and ask the recipient-system administrator or provider to check storage state for the failure time.
Actions by owner
Sender administrator
- Keep retries bounded and provide code, time, and affected-flow identifier after exhaustion.
Recipient administrator
- Restore available mail-system capacity and verify a new attempt can be accepted.
Provider
- Inspect managed storage use and limits, then preserve the exact status after remediation.
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.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
Last verified:
