title: what's the issue with Message-Id in email sent by php
body: <p>I have suspicious message-id header of email sent by php to gmail account:</p>
  
  <pre><code>Message-Id: &lt;5100054f.a489440a.5d93.6a70SMTPIN_ADDED_MISSING@mx.google.com&gt;
  </code></pre>
  
  <p>Could you please tell does it have this strange format and what SMTPIN_ADDED_MISSING means here? Examples I saw in the internet had format something like this containing sending domain but my message id doesn't contain it for some reason:</p>
  
  <pre><code>38D1C1FD-3C35-4568-925C-FC46CAC0DE8A@sendinghost.com
  </code></pre>
  
  <p>I don't think I set this header in Zend_Mail. What generates this headers? Do you see any issues with this header?</p>