title: How does the email header field 'thread-index' work?
body: <p>I was wondering if anyone knew how the thread-index field in email headers work?</p>
  
  <p>Here's a simple chain of emails thread indexes that I messaged myself with. </p>
  
  <pre><code>Email 1 Thread-Index: AcqvbpKt7QRrdlwaRBKmERImIT9IDg==
  Email 2 Thread-Index: AcqvbpjOf+21hsPgR4qZeVu9O988Eg==
  Email 3 Thread-Index: Acqvbp3C811djHLbQ9eTGDmyBL925w==
  Email 4 Thread-Index: AcqvbqMuifoc5OztR7ei1BLNqFSVvw==
  Email 5 Thread-Index: AcqvbqfdWWuz4UwLS7arQJX7/XeUvg==
  </code></pre>
  
  <p>I can't seem to say with certainty how I can link these emails together. Normally, I would use the in-reply-to field or references field, but I recently found that Blackberrys do NOT include these fields. The only include Thread-Index field. </p>