title: What do X-headers in mails stand for?
body: <p>I'm wondering what all <code>X-</code> headers in e-mails stand for.</p>
  <ul>
  <li>Are they really just a custom and everyone can make them up?</li>
  <li>Are they documented somewhere? I've quickly browsed <a href="https://www.rfc-editor.org/rfc/rfc5322" rel="nofollow noreferrer">RFC 5322</a>. It does not seem to mention anything about custom headers.</li>
  <li>When or by whom were they introduced?</li>
  </ul>
  <p>Thank you for shedding some light in that field :)</p>