title: X- header missing out on reply
body: <p>I am trying to organize emails in my app. For this purpose I add tried adding a custom header to my outgoing emails. Something like this</p>
  
  <pre><code>X-Discussion-Id: 400
  </code></pre>
  
  <p>The problem is that, I don't receive this on reply. Is there any solution to this?? I have tried the method where we add +number to the reply-to and from. the problem with that approach is that it creates many new contacts for the users and I don't like that.</p>