title: What is the correct content-type and document structure for an email that only contains an text/html part
body: <p>First off- I'm fixing up an existing open source library.  While I know that people SHOULD send a plaintext version of a message when they send an html email, this isn't a best-practices question.  If I don't maintain backwards compatibility, they won't accept my patch.</p>
  <p>I'm trying to figure out how to best handle situations where ONLY an html email is sent.</p>
  <p>The library currently generates this:</p>
  <pre><code>MIME-Version: 1.0
  Content-Type: text/html;
  
  Hello, World
  </code></pre>
  <p>But every html-only message I've seen in my inbox shows:</p>
  <pre><code>MIME-Version: 1.0
  Content-Type: multipart/alternative; 
      boundary=&quot;----=_Part_1935495_1732146301.1367384830372&quot;
  
  ----=_Part_1935495_1732146301.1367384830372
  Content-Type: text/html;
  
  Hello, World
  ----=_Part_1935495_1732146301.1367384830372--
  </code></pre>
  <p>I can't figure out if this is a best-practice or a requirement</p>
  <p>I've been through :</p>
  <ul>
  <li><a href="https://www.rfc-editor.org/rfc/rfc2557" rel="nofollow noreferrer">https://www.rfc-editor.org/rfc/rfc2557</a></li>
  <li><a href="http://www.ietf.org/rfc/rfc2854.txt" rel="nofollow noreferrer">http://www.ietf.org/rfc/rfc2854.txt</a></li>
  </ul>
  <p>but couldn't find any information.</p>