# 450 4.7.1 Requested action not taken: mailbox temporarily unavailable

- **ID:** `communication/smtp-450-requested-action-not-taken`
- **Domain:** communication
- **Category:** network_error
- **Error Code:** `450`
- **Verification:** ai_generated
- **Fix Rate:** 70%

## Root Cause

SMTP server returns a temporary failure (450) due to transient issues like greylisting, DNS lookup failure for the recipient domain, or recipient server rejecting the connection.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| RFC 5321 | active | — | — |
| Postfix 3.8 | active | — | — |
| Exim 4.97 | active | — | — |
| Sendmail 8.17 | active | — | — |

## Workarounds

1. **Implement exponential backoff retry logic in the MTA. Configure Postfix to wait at least 5 minutes before retrying, with max retries of 5. This handles greylisting and transient DNS issues.** (85% success)
   ```
   Implement exponential backoff retry logic in the MTA. Configure Postfix to wait at least 5 minutes before retrying, with max retries of 5. This handles greylisting and transient DNS issues.
   ```
2. **Verify the recipient domain's MX records are resolvable. Use 'dig MX recipientdomain.com' to check. If the MX is missing or returns NXDOMAIN, the 450 is caused by DNS failure, not a mailbox issue.** (80% success)
   ```
   Verify the recipient domain's MX records are resolvable. Use 'dig MX recipientdomain.com' to check. If the MX is missing or returns NXDOMAIN, the 450 is caused by DNS failure, not a mailbox issue.
   ```

## Dead Ends

- **Immediately resend the email without any delay** — Greylisting requires a delay (typically 300-600 seconds) before retry; immediate resend is ignored and returns the same 450 (90% fail)
- **Permanently remove the recipient from the mailing list** — 450 is a temporary failure, not permanent; the mailbox may be available later (95% fail)
- **Change the sender email address to a different domain** — Greylisting is based on the tuple (sender IP, sender email, recipient email); changing only the sender domain may not bypass it if the IP is still greylisted (70% fail)