EC
450
SMTPWARNNotableTemporary Delivery FailureHIGH confidence

Requested mail action not taken: mailbox unavailable

What this means

The 450 code is a temporary failure stating the mailbox is unavailable, but the client should try again later. This can happen if the mailbox is locked, temporarily disabled, or the server is busy.

Why it happens
  1. 1The recipient's mailbox is locked by another process (e.g., another session retrieving mail).
  2. 2The mail server is currently too busy to process the request.
  3. 3The recipient's account has a temporary issue, like being over a soft quota.
How to reproduce

A client tries to deliver a message while the recipient's mailbox is being accessed by a POP3 session.

trigger — this will error
trigger — this will error
RCPT TO:<busy-user@example.com>
450 4.2.1 Mailbox temporarily unavailable

expected output

450 Mailbox unavailable

Fix

Retry delivery later

WHEN When operating a mail server or sending client

Retry delivery later
// Queue the message for a later delivery attempt
if (smtpCode === 450) {
  queueForRetry(message, { interval: '5m' });
}

Why this works

This is a transient error, and retrying is the correct course of action.

What not to do

✕ Treat as a permanent failure

This is a temporary condition, and treating it as a hard bounce (5xx) would be incorrect and prevent successful delivery later.

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