Mailgun – Case: Sending error

• 事象
  • mod.go.jp → simisin.netへのメールは届くが、simisin.net → mod.go.jp のメールは不着になる
  • 前提
    • simisin.net
      • MX: Cloudflare→gmail
      • SMTP: Mailgun
• 調査
  • ログ調査の結果、unable to establish a TLS connection
    • MX (mail1.mod.go.jp)がTLS接続に対応していない
• 対応
  • MailgunのTLS Connectionの設定を”Required”→”Opportunistic (default)”に緩和
    • これで送信できるようになった
• 参考
  • https://documentation.mailgun.com/en/latest/user_manual.html#sending-via-api

{
	"severity": "permanent",
	"tags": [],
	"timestamp": 1667989372.289449,
	"storage": {
		"url": "https://storage-us-east4.api.mailgun.net/v3/domains/simisin.net/messages/AwABBRZxKUTaAVyX[deleted]",
		"region": "us-east4",
		"key": "[deleted]",
		"env": "production"
	},
	"delivery-status": {
		"mx-host": "mail1.mod.go.jp",
		"attempt-no": 1,
		"description": "",
		"session-seconds": 1.7871699333190918,
		"enhanced-code": "",
		"code": 554,
		"message": "unable to establish a TLS connection"
	},
	"recipient-domain": "inet.gsdf.mod.go.jp",
	"id": "aFu8-4YXQJaNrpfSh3UjVA",
	"campaigns": [],
	"reason": "generic",
	"user-variables": {},
	"flags": {
		"is-routed": false,
		"is-authenticated": true,
		"is-system-test": false,
		"is-test-mode": false
	},
	"log-level": "error",
	"envelope": {
		"transport": "smtp",
		"sender": "[deleted]@simisin.net",
		"sending-ip": "159.135.228.12",
		"targets": "[deleted]@inet.gsdf.mod.go.jp"
	},
	"message": {
		"headers": {
			"to": "●●● <[deleted]@gmail.com>, [deleted]@inet.gsdf.mod.go.jp",
			"message-id": "CAFHVER=KVUExDhAVq_0pq14ZnyQo5mXrWMbVN6sj1Qz-93EStg@mail.gmail.com",
			"from": "[deleted] <[deleted]@simisin.net>",
			"subject": "Re: ●●●●●●●●"
		},
		"attachments": [
			{
				"filename": "●●●●●●.xlsx",
				"content-type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
				"size": 35896
			}
		],
		"size": 50766
	},
	"recipient": "[deleted]@inet.gsdf.mod.go.jp",
	"event": "failed"
}