Gmailのスパムフィルターを通り抜けるViagra

私は、いくつかあるメールアカウントのメールを全てGmailに転送しています。その中のほとんどのスパムメールはGmailのフィルターで迷惑メールとして振り分けられるのですが、以下の様な私のメールアドレスを偽装した「Approved VIAGRA® Store」と名乗るメールが頻繁に紛れ込んできます。しかし、同じ様なメールでもちゃんと迷惑メールのフォルダに振り分けられるものもあるので、なぜ一部のメールだけフィルターされないのか原因が判りません。

Delivered-To: myaddress@gmail.com

Received:

by 10.220.91.31

with SMTP id k31cs64591vcm;

Sat, 13 Mar 2010 00:49:48 -0800 (PST)

Received:

by 10.142.4.13

with SMTP id 13mr3368206wfd.65.1268470187704;

Sat, 13 Mar 2010 00:49:47 -0800 (PST)

Return-Path: <otheraddress@xxx.biglobe.ne.jp>

Received: from rcpt-expgw2.biglobe.ne.jp (rcpt-expgw2.biglobe.ne.jp [122.130.255.2])

by mx.google.com

with ESMTP id 10si4551047pzk.113.2010.03.13.00.49.47;

Sat, 13 Mar 2010 00:49:47 -0800 (PST)

Received-SPF: pass (google.com: domain of otheraddress@xxx.biglobe.ne.jp designates 122.130.255.2 as permitted sender) client-ip=122.130.255.2;

Authentication-Results: mx.google.com; spf=pass (google.com: domain of otheraddress@xxx.biglobe.ne.jp designates 122.130.255.2 as permitted sender) smtp.mail=otheraddress@xxx.biglobe.ne.jp

Received: from biglobe.ne.jp

by rcpt-expgw2.biglobe.ne.jp (shby/5910021009)

with SMTP id o2D8nkUl020641 for <myaddress@gmail.com>;

Sat, 13 Mar 2010 17:49:46 +0900

Received: from rcpt-impgw.biglobe.ne.jp

by biglobe.ne.jp (RCPT_GW) id RAA00732;

Sat, 13 Mar 2010 17:49:46 +0900 (JST)

Received: from 93-36-105-160.ip59.fastwebnet.it (93-36-105-160.ip59.fastwebnet.it [93.36.105.160])

by rcpt-impgw.biglobe.ne.jp (shby/1714221009)

with SMTP id o2D8nBKi031207;

Sat, 13 Mar 2010 17:49:44 +0900

Authentication-Results: rcpt-impgw.biglobe.ne.jp smtp.mail=otheraddress@xxx.biglobe.ne.jp; spf=permerror

Date: Sat, 13 Mar 2010 17:49:11 +0900

Message-Id: <201003130849.o2D8nBKi031207@rcpt-impgw.biglobe.ne.jp>

From: Approved VIAGRA® Store <myaddress@xxx.biglobe.ne.jp>

Subject: Important notice: Google Apps browser support

To: <myaddress@xxx.biglobe.ne.jp>

MIME-Version: 1.0

Content-Type: text/html

X-Biglobe-VirusCheck: Sat, 13 Mar 2010 17:49:46 +0900