email - GMail が HTML ではなくプレーンテキストのメールを表示する

Question

私の Rails 3 アプリケーションは、プレーンテキスト形式と HTML 形式の両方で電子メールを送信します。RoundCube と Squirrel Mail クライアントを使用してローカルでテストしましたが、どちらも画像やリンクなどを含む HTML バージョンを表示します。一方、GMail はプレーンテキスト形式を選択します。これの原因は何ですか？

Received-SPF: neutral (google.com: 69.123.123.123 is neither permitted nor denied by best guess record for domain of info@example.com) client-ip=69.123.123.123;
Date: Fri, 04 Mar 2011 01:01:45 +0000
From: info@example.com
To: test@gmail.com
Message-ID: <4d7039f9e9d3e_3449482ab7831658@test.mail>
Subject: Your example account was activated.
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_4d7039f9e6967_3449482ab7831370";
 charset=UTF-8
Content-Transfer-Encoding: 7bit

----==_mimepart_4d7039f9e6967_3449482ab7831370
Date: Fri, 04 Mar 2011 01:01:45 +0000
Mime-Version: 1.0
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Content-ID: <4d7039f9e95ed_3449482ab7831519@test.mail>

<html>
  <head>
    <meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
  </head>
  <body>
    <p><a href="http://example.com/"><img border="0" src="http://example.com/images/logo.png" alt="example logo" /></a></p>
    <p>Congratulations, Test!</p>
    <p>
      Your <a style="text-decoration:none;color:#ef4923;" href="http://example.com/">example</a> account was activated.
    </p>
  </body>
</html>

----==_mimepart_4d7039f9e6967_3449482ab7831370
Date: Fri, 04 Mar 2011 01:01:45 +0000
Mime-Version: 1.0
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Content-ID: <4d7039f9e8b0e_3449482ab78314b7@test.mail>

Congratulations, Test!

Your example.com account was activated.

----==_mimepart_4d7039f9e6967_3449482ab7831370--

score 101 · Accepted Answer

HTML 部分をプレーンテキスト部分の後に配置して、メッセージの部分の順序を入れ替えてみてください。それはうまくいくかもしれません:)。

注：これをどこで読んだか（または確かに読んだかどうか）を今思い出せませんが、切り替えが役立つ理由は、メッセージの優先部分が最後の部分である可能性があるためです。

更新: マルチパート MIME メッセージのパーツは優先度の高い順に並べる必要があると書かれている場所を見つけました --ここ、セクション 7.2.3 (編集: 最新バージョンはこちら; @ALEXintlsosに感謝します!) で、3 番目から最後の段落。

更新: セクション 7.2.3 の引用を次に示します ( https://stackoverflow.com/help/referencingを参照)。

7.2.3 The Multipart/alternative subtype
The multipart/alternative type is syntactically identical to multipart/mixed, 
but the semantics are different. In particular, each of the parts is an
"alternative" version of the same information. User agents should recognize
that the content of the various parts are interchangeable. The user agent
should either choose the "best" type based on the user's environment and
preferences, or offer the user the available alternatives. In general, choosing
the best type means displaying only the LAST part that can be displayed. This
may be used, for example, to send mail in a fancy text format in such a way
that it can easily be displayed anywhere:

From:  Nathaniel Borenstein <nsb@bellcore.com> 
To: Ned Freed <ned@innosoft.com> 
Subject: Formatted text mail 
MIME-Version: 1.0 
Content-Type: multipart/alternative; boundary=boundary42 


--boundary42 
Content-Type: text/plain; charset=us-ascii 

...plain text version of message goes here.... 

--boundary42 
Content-Type: text/richtext 

.... richtext version of same message goes here ... 
--boundary42 
Content-Type: text/x-whatever 

.... fanciest formatted version of same  message  goes  here 
... 
--boundary42-- 

In this example, users whose mail system understood the "text/x-whatever"
format would see only the fancy version, while other users would see only the
richtext or plain text version, depending on the capabilities of their system.

In general, user agents that compose multipart/alternative entities should
place the body parts in increasing order of preference, that is, with the
preferred format last. For fancy text, the sending user agent should put the
plainest format first and the richest format last. Receiving user agents should
pick and display the last format they are capable of displaying. In the case
where one of the alternatives is itself of type "multipart" and contains
unrecognized sub-parts, the user agent may choose either to show that 
alternative, an earlier alternative, or both.

NOTE: From an implementor's perspective, it might seem more sensible to reverse
this ordering, and have the plainest alternative last. However, placing the
plainest alternative first is the friendliest possible option when
multipart/alternative entities are viewed using a non-MIME- compliant mail
reader. While this approach does impose some burden on compliant mail readers,
interoperability with older mail readers was deemed to be more important in
this case.

It may be the case that some user agents, if they can recognize more than one
of the formats, will prefer to offer the user the choice of which format to
view. This makes sense, for example, if mail includes both a nicely-formatted
image version and an easily-edited text version. What is most critical, however,
is that the user not automatically be shown multiple versions of the same data.
Either the user should be shown the last recognized version or should 
explicitly be given the choice.

email - GMail が HTML ではなくプレーン テキストのメールを表示する

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email - GMail が HTML ではなくプレーンテキストのメールを表示する