4.3.2System temporarily not accepting network messages

The host on which the recipient's mailbox resides is not currently accepting messages. Because the code begins with 4, this is a temporary failure and delivery may be retried in a controlled way.

The X.3.2 pattern means that the host on which the mailbox resides is not accepting messages. The standard lists an imminent shutdown, excessive load, and system maintenance as examples. The pattern itself can describe either a permanent or a persistent transient error; in the concrete 4.3.2 code, the leading digit assigns the result to the transient class.

The leading digit 4 denotes a persistent transient failure: the current attempt failed, but the condition may clear. The code does not by itself mean that the recipient address is invalid or establish that the host is permanently unable to accept mail.

Class

Temporary failure

Retry

Controlled retry

Suppression

Check the full context

Operational recommendation: check suppression state before every attempt, then retry with backoff, jitter, idempotency, and a time or attempt limit. Stop after success, a permanent result, or exhaustion of the limit; refer repeated 4.3.2 results to the recipient-system administrator or provider instead of retrying indefinitely.

Operational recommendation: do not add the address to a suppression list based on temporary code 4.3.2 alone. Inspect the full response, provider context, and bounded-retry history; suppress only after an independent permanent signal or when the applicable policy requires it.

• The mailbox host is approaching shutdown and is temporarily not accepting messages.
• The host is excessively loaded or undergoing system maintenance.
• In the accepted Microsoft 365 / Exchange Online example, the system reports that the recipient thread limit was exceeded; this is a documented provider case, not the general definition of 4.3.2.

1. Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.3.2 and that the basic reply is in the 4xx class.
2. Correlate the response with the intended message, attempt time, and host that returned it; retain the full text and any provider subcode because 4.3.2 alone does not identify the particular cause.
3. Ask the recipient-system administrator or provider to inspect shutdown state, load, and maintenance for the same time; if the response exactly matches the Microsoft example, also inspect the reported recipient-thread-limit condition.
4. Compare the results of bounded retries and stop them after success, a permanent rejection, or the configured limit is reached.

An accepted Tier-1 example from Microsoft 365 / Exchange Online reads exactly: "432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded". It documents a specific Microsoft practice — a 432 reply with 4.3.2 and the STOREDRV.Deliver subcode for an exceeded recipient thread limit — but does not change the broader standard meaning of X.3.2. Retry and suppression recommendations remain dependent on operational context.

432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded

Compare 4.3.0 for another or undefined mail-system problem and 4.3.1 for exhausted mail-system storage. These distinctions help separate a host that is not accepting messages from a general system condition and storage exhaustion.

• 4.3.0Other or undefined temporary mail-system status
• 4.3.1Temporarily full mail system

The standard meaning of X.3.2, its class assignment, and interpretation rules were verified against the IANA registry and RFC 2034, RFC 3463, RFC 5248, and RFC 7504 as of July 17, 2026. The exact provider example comes from accepted Microsoft record microsoft-ph02-014; retry, diagnostic, and suppression guidance is presented separately as operational advice.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.3.2

• rfc5248T0 source

  Section 2.1: registry fields and non-exclusive Associated Basic Status Code

• rfc2034T0 source

  Section 4: enhanced status class agrees with SMTP reply class

• rfc3463T0 source

  IANA registry reference for X.3.2

• rfc7504T0 source

  IANA registry reference for X.3.2

• microsoft-ph02-014T1 source

  Common error codes in nondelivery reports table > row 432 4.3.2

Last verified: 17 July 2026