早速のご回答、ありがとうございます。

リンク先を見させていただきました。時間的には書かれていたことが原因のようです（メールが来なくなった時刻にも当てはまります）。
本当は問い合わせをする前に調べられることだったのに、恥ずかしい限りで、ぜんの様にはご迷惑をかけました。

一応、メッセージヘッダは以下の通りです。

Return-Path: <alerts-error@reply.yahoo.co.jp>
Delivered-To: ******@nakayoshi.cc
Received: (qmail 45835 invoked from network); 22 May 2007 02:28:00 +0900
Received: from unknown (HELO mgw2.zenno.net) (192.168.0.147)
by mail2b.zenno.net with SMTP; 22 May 2007 02:28:00 +0900
Received: (qmail 68479 invoked from network); 22 May 2007 02:28:00 +0900
Received: from n19.bulk.tnz.yahoo.co.jp (203.216.240.88)
by mgw2.zenno.net with SMTP; 22 May 2007 02:28:00 +0900
Comment: DomainKeys? See http://antispam.yahoo.com/domainkeys
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=dk200510; d=mail.yahoo.co.jp;
b=lJt6aES1f30ExwbQob/fAMVB9tIX9hpyGYA3InrJofTYnWue8x/5nzWkt4NLF7Sm5tViM1Fb7mZOvtBq+x7o1nHblvKeUTYirHpQ4k0+UILpEkzLKZEb5YB0xvs8B8FqvcP6o8Fvhzhg5GUEre9cHh9+T2SZxW2PSjaXUXuRfbM=;
Received: from [203.216.229.208] by n19.bulk.tnz.yahoo.co.jp with NNFMP; 21 May 2007 17:28:00 -0000
Received: from [203.216.241.8] by mailer07.bulk.tnz.yahoo.co.jp with NNFMP; 21 May 2007 17:28:00 -0000
Date: 22 May 2007 02:28:00 +0900
Received: from [127.0.0.1] by dlv03.alerts.tnz.yahoo.co.jp with NNFMP; 21 May 2007 17:28:00 -0000
X-yahoo-newman-property: alerts
X-yahoo-newman-id: Y030002ba962a-******@nakayoshi.cc
From: "Yahoo! Chiebukuro Alerts" <alerts-chiebukuro@mail.yahoo.co.jp>
To: ******@nakayoshi.cc
Reply-To: "Yahoo! Chiebukuro Alerts" <alerts-chiebukuro@mail.yahoo.co.jp>
Errors-To: alerts-error@reply.yahoo.co.jp
Return-Path: alerts-error@reply.yahoo.co.jp
Subject: =?iso-2022-jp?B?WWFob28hIBskQkNON0NCXiUiJWkhPCVIGyhCIC0gGyRCJCIkSiQ/GyhC?=
=?iso-2022-jp?B?GyRCJE4yc0V6JDckPzxBTGQkRyEiJFskKyROMnNFeiQsJVklOSVIGyhC?=
=?iso-2022-jp?B?GyRCJSIlcyU1ITwkS0EqJFAkbCReJDckPyEjGyhC?=
X-Yalerts-EncodeEmail: iso-2022-jp
X-Yahoo-AlertId:F5vyfaI1CiJPMfSqru2nKOx3I4IiOhVJXRswwgAF8w4kDxtcUFIeWSRUf9DOnu.5M5fTf4QBFQRJyNiolZ7HPrdf
X-Yahoo-Alerts-DLV-Time: Tuesday, May 22, 2007, 2:28 AM JST
Content-Type: text/plain; charset="iso-2022-jp"
Content-Transfer-Encoding: 7bit
X-NAS-BWL: No match found for 'alerts-chiebukuro@mail.yahoo.co.jp' (127 addresses, 0 domains)
X-NAS-Bayes: #0: 0; #1: 1
X-NAS-Classification: 0
X-NAS-MessageID: 1476
X-NAS-Validation: {CBE6F8E5-DC11-4D25-B94A-5F5B36CB9636}

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