# 450 4.7.1 请求的操作未执行：邮箱暂时不可用

- **ID:** `communication/smtp-450-requested-action-not-taken`
- **领域:** communication
- **类别:** network_error
- **错误码:** `450`
- **验证级别:** ai_generated
- **修复率:** 70%

## 根因

SMTP服务器因临时问题返回450错误，如灰名单、收件人域DNS查找失败或收件人服务器拒绝连接。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| RFC 5321 | active | — | — |
| Postfix 3.8 | active | — | — |
| Exim 4.97 | active | — | — |
| Sendmail 8.17 | active | — | — |

## 解决方案

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.
   ```
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.
   ```

## 无效尝试

- **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% 失败率)
- **Permanently remove the recipient from the mailing list** — 450 is a temporary failure, not permanent; the mailbox may be available later (95% 失败率)
- **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% 失败率)