title: Multiple In-Reply-To
body: <p>Mail user agents usually display threads of Emails by chaining messages together according to the <em>In-Reply-To</em> and <em>References</em> header fields that contain the <em>Message-ID</em>s of other messages. Although a mail usually only replies to one other message, it may be the case that one message answers multiple others. <a href="https://www.rfc-editor.org/rfc/rfc5322" rel="nofollow noreferrer">The standard</a> allows multiple entries in both fields. What can I expect when I send an email that <em>References</em> or is <em>In-Reply-To</em> multiple IDs this way?</p>
  <ul>
  <li>Is it good practice to do so?</li>
  <li>Does it confuse widespread MUAs?</li>
  <li>Is there any common ground on how to display such a message in a
  threaded view?</li>
  </ul>