The short answer

SMTP 431 is a temporary rejection meaning the receiving server is low on disk, memory, or queue space. It comes mainly from Microsoft Exchange and Windows SMTP servers, not from RFC 5321 (which uses 452 for this). Queue and let your sender retry; a persistent 431 is the recipient server admin's problem.

Quick fix

Retry later with backoff.

4 - Temporary. Safe to retry. The message is not lost and most servers retry automatically.

What it means

SMTP 431 usually reads "431 4.3.1 STOREDRV; mailbox disk is full" or "431 4.3.1 STOREDRV.Submit.Exception" on Exchange. The leading 4 makes it transient, so the message is not permanently rejected and should be queued and retried. This is the receiving server saying it is out of resources. It is not caused by too many recipients, and nothing in your message or recipient list can be edited to make it go away.

Common causes

• Store-level disk-full check (STOREDRV)

  Exchange checks free space on the drive holding the mailbox database, transaction logs, or system TEMP folder. Below roughly 1GB it rejects with "431 4.3.1 STOREDRV; mailbox disk is full", logged as a SUBMITDEFER event.

• Transport back pressure

  Exchange separately monitors disk, memory, and queue-database usage as percentages. When a resource crosses a threshold it throttles inbound mail and answers 431/452 4.3.1, logging Resource Manager Event IDs 15004-15007.

How to fix it

1. Do nothing first, let it retry
   Because 431 is transient, a well-behaved mail server requeues and retries with backoff. Many 431s clear once the remote server frees resources after a spike or maintenance.

2. Confirm it is not being hard-bounced
   Check your logs. A 431 should defer, not bounce. If your platform converts it to a permanent failure, fix the retry handling.

3. If it persists for hours, contact the recipient admin
   The fix lives on their server (free disk, add memory, relieve back pressure). You cannot resolve it by editing your message.

4. If you run the receiving Exchange server
   For a STOREDRV/SUBMITDEFER event, free disk on the database, log, or TEMP volume. For back pressure (Event IDs 15004-15007), check disk, memory, and queue-database utilization and free space on the transport-queue and database volumes.

Paired code

A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:

• X.3.1Mail system full

Common questions

• Is 431 permanent or temporary?
  Temporary. The leading 4 puts it in the transient class, so it should be queued and retried, not bounced. It clears once the receiving server recovers its resources.

• Is 431 a real RFC code?
  No. RFC 5321 expresses this condition as 452. 431 is a vendor extension used mainly by Microsoft Exchange and the Windows SMTP service.

• Does 431 mean too many recipients?
  No. It means the receiving server is out of system resources (disk, memory, or queue space).

Related codes

• 452Insufficient system storage or too many recipients.

• 451Requested action aborted — local error in processing.

• 450Mailbox unavailable (temporary).

• 421Service not available, closing channel.

• 552Storage allocation exceeded / message too large.

• All SMTP error codes

Most rejections come back to sender reputation.

Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.