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450

Mailbox Unavailable

The recipient's mailbox is temporarily unavailable. This is often due to greylisting or server policies.

What does this error mean?

SMTP error 450 indicates a temporary condition where the mailbox cannot receive mail right now:

  • Greylisting - intentional delay for spam prevention
  • Mailbox is temporarily locked
  • Policy-based temporary rejection
  • Server is asking you to retry later

Common Causes

  • hourglass_empty
    Greylisting

    Server delays first-time senders to verify they're legitimate mail servers that will retry.

  • lock
    Mailbox Locked

    The mailbox is temporarily locked due to maintenance or quota operations.

  • policy
    Policy Restriction

    Temporary policy-based rejection, often related to sending patterns.

    • How to Fix

    1. Wait and Retry Automatically

    Your mail server should automatically retry. Greylisting typically accepts messages after 5-15 minutes.

    2. Configure Proper Retry Settings

    Ensure your mail server is configured to retry temporary failures with appropriate delays.

    3. Check for Pattern Issues

    If errors persist, review your sending patterns and ensure you're not triggering rate limits.

    Example response 
    450 4.2.1 Mailbox temporarily disabled
450 4.7.1 Greylisted, please retry in 5 minutes
450 Requested mail action not taken: mailbox unavailable

    Related SMTP Errors

    421 Service Not Available 451 Local Error 550 Mailbox Not Found (Permanent) 452 Insufficient Storage

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