450 communication network_error ai_generated partial

450 4.7.1 Requested action not taken: mailbox temporarily unavailable

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

Also available as: JSON · Markdown · 中文
70%Fix Rate
83%Confidence
1Evidence
2024-05-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
RFC 5321 active — — —
Postfix 3.8 active — — —
Exim 4.97 active — — —
Sendmail 8.17 active — — —

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.

generic

中文

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。

Official Documentation

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

Workarounds

  1. 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. 
    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. 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. 
    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.

中文步骤

  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.

Dead Ends

Common approaches that don't work:

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

    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% fail

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

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

    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