450 communication network_error ai_generated partial

450 4.7.1 请求的操作未执行：邮箱暂时不可用

450 4.7.1 Requested action not taken: mailbox temporarily unavailable

ID: communication/smtp-450-requested-action-not-taken

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70%修复率
83%置信度
1证据数
2024-05-05首次发现

版本兼容性

版本状态引入弃用备注
RFC 5321 active — — —
Postfix 3.8 active — — —
Exim 4.97 active — — —
Sendmail 8.17 active — — —

根因分析

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。

English

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.

generic

官方文档

https://datatracker.ietf.org/doc/html/rfc5321#section-4.2.5

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

常见但无效的做法:

  1. Immediately resend the email without any delay 90% 失败

    Greylisting requires a delay (typically 300-600 seconds) before retry; immediate resend is ignored and returns the same 450

  2. Permanently remove the recipient from the mailing list 95% 失败

    450 is a temporary failure, not permanent; the mailbox may be available later

  3. Change the sender email address to a different domain 70% 失败

    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