title: Understanding email headers: x-beenthere and x field
body: <p>I'd like to understand more about the email headers.</p>
  <p>After looking at one of the emails, I found the field had this entry &quot;x-BeenThere&quot; in the header. What does this mean?</p>
  <p>Also I noticed some &quot;x&quot; fields seem like they're custom set fields, is this the case with some emails?</p>