2005 10 17 Delivery Status Notification (Failure) 18 failure notice 19 Mail delivery failed: returning message to sender 20 Mail System Error - Returned Mail 21 Returned mail: see transcript for details 22 Returned mail: User unknown 23 Undeliverable Mail 24 Undeliverable message 25 Undelivered Mail Returned to Sender 26 メッセージを配信できません