TL;DR
The recipient host is temporarily refusing messages. Retry with limits; do not suppress until independent permanent evidence appears.
What this code means
X.3.2 means the mailbox host is not accepting messages. 4.3.2 is temporary and can cover shutdown, load, or maintenance.
Provider examples
432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. example.com. PRX8 [example.com]421 4.3.2 Service not available [outlook.com]451 4.3.2 Temporary server error. Please try again later ATTR17 [outlook.com]smtp;451 4.3.2 Internal server errorTechnical meaning
The full response may narrow the cause; the generic code alone cannot identify the host condition.
Delivery status
Class 4 permits controlled retry and does not establish permanent host unavailability.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Check suppression, then retry with idempotency, backoff, jitter, and a time or attempt limit.
Suppression decision
Do not suppress on 4.3.2 alone; use independent permanent evidence or policy.
Common causes
- Host shutdown, excessive load, or maintenance.
Diagnostic steps
- Retain host, time, full response, and subcode; ask the administrator or provider to inspect state and load.
Actions by owner
Sender administrator
- Group results by host and time, and stop retrying after the configured limit.
Recipient administrator
- Clear confirmed host conditions and verify message acceptance.
Provider
- Inspect availability, load, and response-specific limits, preserving code and subcode.
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.
- RFC 7504 — SMTP 521 and 556 Reply Codes — Defines SMTP reply codes for hosts that do not accept mail.
- Email nondelivery reports and SMTP errors in Exchange Online — Microsoft Learn overview of Exchange Online NDR and SMTP errors.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
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