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SMTP Error 450: Mailbox Unavailable

SMTP error 450 indicates that the requested mail action was not taken because the mailbox is temporarily unavailable. This is a temporary failure that typically occurs when the mailbox is locked, busy, or experiencing issues.

What Does 450 Mean?

The SMTP reply code 450 follows the standard SMTP response format:

  • 4 = Temporary failure (transient negative completion reply)
  • 50 = Requested mail action not taken: mailbox unavailable

When you receive a 450 error, it means the receiving mail server cannot access the recipient's mailbox at this moment, but the issue is likely temporary. You should retry sending the email later.

Error Type

  • Type: Temporary failure (soft error)
  • Category: Mailbox availability
  • Action Required: Retry sending after a delay

Common Causes

  1. Mailbox Locked: The mailbox is locked by another process or user
  2. Mailbox Busy: The mailbox is currently being accessed or modified
  3. Quota Exceeded Temporarily: The mailbox has exceeded its quota, but space may become available
  4. Server Maintenance: The mail server is performing maintenance on the mailbox
  5. Database Issues: Temporary database connectivity or locking issues
  6. Account Suspension: The account is temporarily suspended (may become permanent if not resolved)
  7. Mailbox Migration: The mailbox is being migrated to another server

How to Resolve

For Email Senders

  1. Wait and Retry: Wait 15-30 minutes before retrying the send
  2. Check Recipient Status: Verify if the recipient's email account is active
  3. Reduce Send Rate: If sending to multiple recipients, reduce your sending rate
  4. Contact Recipient: If the error persists, contact the recipient to check their mailbox status
  5. Monitor Retry Success: Track whether retries eventually succeed

For Developers

  1. Implement Retry Logic: Add automatic retry with delays (15-30 minutes)
  2. Queue Management: Queue failed messages for retry rather than failing immediately
  3. Error Tracking: Log 450 errors separately to track mailbox availability issues
  4. Rate Limiting: Implement rate limiting to avoid overwhelming mailboxes
  5. Status Monitoring: Monitor mailbox status and adjust retry strategies

Related SMTP Errors

Examples

Example Error Message

450 Requested mail action not taken: mailbox unavailable

Example with Enhanced Status Code

450 4.2.0 Mailbox temporarily unavailable

Common Email Provider Responses

  • Exchange: "450 4.2.0 Mailbox temporarily unavailable"
  • Postfix: "450 4.2.0 Mailbox temporarily unavailable"
  • Custom Servers: Varies by mail server configuration

Best Practices

  1. Always Retry: 450 errors should always be retried after a delay
  2. Use Appropriate Delays: Wait 15-30 minutes before retrying
  3. Set Maximum Retries: Limit retry attempts (e.g., 3-5 attempts over 24 hours)
  4. Monitor Success Rate: Track how many 450 errors eventually succeed on retry
  5. Distinguish from Permanent Errors: Don't treat 450 as a permanent failure
  6. Queue for Retry: Use message queues to handle retries efficiently