title: What's "X-KeepSent" header in email?
body: <p>We received an email with unknown header :</p>
  
  <pre><code>...
  Subject: =?ISO-8859-1?Q?Gestion_des_impay=E9s_=3A_fichiers_du_03=2F04=2F2012?=
  MIME-Version: 1.0
  X-KeepSent: 41247AD0:C9969B98-C12579D5:004A3203;
   type=4; name=$KeepSent
  Message-ID: &lt;OF41247AD0.C9969B98-ONC12579D5.004A3203-C12579D5.004A3987@bnpparibas.com&gt;
  From: ...
  </code></pre>
  
  <p>My question is what's this [X-KeepSent] header ???
  This header seems to create some problems.</p>