title: Do all email clients use "In-Reply-To" field in email header?
body: <p>When any email is send with a Message-ID header, then any replies to it include In-Reply-To naming that ID and References which can name a list of parent message id's, and email clients use this information to construct threads when viewing a list of emails in threaded view.</p>
  
  <p>My question is: Do all email clients (Outlook, Gmail, Exchange, etc) append In-Reply-To field in email header while sending reply to any existing email?</p>
  
  <p>I referred <a href="https://www.ietf.org/rfc/rfc2822.txt" rel="noreferrer">RFC 2822</a> but it says In-Reply-To field is optional.</p>
  
  <pre><code>Though optional, every message SHOULD have a "Message-ID:" field.
  Furthermore, reply messages SHOULD have "In-Reply-To:"
  </code></pre>
  
  <p>Example of In-Reply-To email header:</p>
  
  <p>First email:</p>
  
  <pre><code>Message-ID &lt;foobar-1234-0@server.com&gt;
  In-Reply-To: not provided
  References: not provided
  </code></pre>
  
  <p>Second email:</p>
  
  <pre><code>Message-ID &lt;foobar-1234-1@server.com&gt;
  In-Reply-To: &lt;foobar-1234-0@server.com&gt;
  References: &lt;foobar-1234-0@server.com&gt;
  </code></pre>
  
  <p>Third email:</p>
  
  <pre><code>Message-ID &lt;foobar-1234-2@server.com&gt;
  In-Reply-To: &lt;foobar-1234-1@server.com&gt;
  References: &lt;foobar-1234-0@server.com&gt; &lt;foobar-1234-1@server.com&gt;
  </code></pre>