{
  "id": "communication/smtp-421-service-not-available",
  "signature": "421 4.7.0 Service not available, closing transmission channel",
  "signature_zh": "421 4.7.0 服务不可用，关闭传输通道",
  "regex": "421\\s+4\\.7\\.0\\s+Service not available",
  "domain": "communication",
  "category": "network_error",
  "subcategory": null,
  "root_cause": "SMTP server rejects connection due to temporary resource exhaustion (e.g., too many concurrent connections, rate limiting, or DNS lookup failures) as defined in RFC 5321 section 4.2.5.",
  "root_cause_type": "generic",
  "root_cause_zh": "SMTP服务器因临时资源耗尽（如并发连接过多、速率限制或DNS查找失败）而拒绝连接，如RFC 5321第4.2.5节所定义。",
  "versions": [
    {
      "version": "Postfix 3.8.0",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Sendmail 8.17.1",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Exim 4.96",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Microsoft Exchange 2019",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Amazon SES SMTP endpoint",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    }
  ],
  "os_specific": {},
  "dead_ends": [
    {
      "action": "Disable rate limiting on the SMTP server entirely",
      "why_fails": "Disabling rate limiting removes protection against spam and DoS attacks; it may cause the server to crash or be blacklisted.",
      "fail_rate": 0.95,
      "condition": "",
      "sources": []
    },
    {
      "action": "Increase the SMTP server's max message size",
      "why_fails": "The 421 error is not about message size but about resource availability; increasing message size can exacerbate the resource issue.",
      "fail_rate": 0.85,
      "condition": "",
      "sources": []
    },
    {
      "action": "Use a different SMTP port (e.g., 587 instead of 25)",
      "why_fails": "Port 587 is for message submission and may have different rate limits, but the underlying resource exhaustion on the server remains unchanged.",
      "fail_rate": 0.7,
      "condition": "",
      "sources": []
    }
  ],
  "workarounds": [
    {
      "action": "Implement exponential backoff retry in the SMTP client: after receiving 421, wait 30 seconds, then 60, then 120, etc., up to a max of 5 retries.",
      "success_rate": 0.85,
      "how": "Implement exponential backoff retry in the SMTP client: after receiving 421, wait 30 seconds, then 60, then 120, etc., up to a max of 5 retries.",
      "condition": "",
      "sources": []
    },
    {
      "action": "Reduce the number of concurrent SMTP connections from your application by adding a connection pool with a max limit (e.g., 10 connections).",
      "success_rate": 0.8,
      "how": "Reduce the number of concurrent SMTP connections from your application by adding a connection pool with a max limit (e.g., 10 connections).",
      "condition": "",
      "sources": []
    },
    {
      "action": "Check the SMTP server logs for 'too many connections' and adjust the 'smtpd_client_connection_count_limit' in Postfix (default 50) to a higher value like 100.",
      "success_rate": 0.75,
      "how": "Check the SMTP server logs for 'too many connections' and adjust the 'smtpd_client_connection_count_limit' in Postfix (default 50) to a higher value like 100.",
      "condition": "",
      "sources": []
    }
  ],
  "workarounds_zh": [
    "Implement exponential backoff retry in the SMTP client: after receiving 421, wait 30 seconds, then 60, then 120, etc., up to a max of 5 retries.",
    "Reduce the number of concurrent SMTP connections from your application by adding a connection pool with a max limit (e.g., 10 connections).",
    "Check the SMTP server logs for 'too many connections' and adjust the 'smtpd_client_connection_count_limit' in Postfix (default 50) to a higher value like 100."
  ],
  "transition_graph": {
    "leads_to": [],
    "preceded_by": [],
    "frequently_confused_with": []
  },
  "official_doc_url": "https://www.rfc-editor.org/rfc/rfc5321#section-4.2.5",
  "official_doc_section": null,
  "error_code": "421",
  "verification_tier": "ai_generated",
  "confidence": 0.86,
  "fix_success_rate": 0.8,
  "resolvable": "partial",
  "first_seen": "2024-01-20",
  "last_confirmed": "2024-06-01",
  "last_updated": "2024-06-01",
  "evidence_count": 1,
  "tags": [],
  "locale": "en",
  "aliases": []
}