title: What is the unique identification for emails?
body: <p>I'm trying to find the guaranteed unique identifier to distinguish emails from each other. Currently, I am writing an application which connects to Microsoft Exchange, however I want to be able to support other email services.</p>
  
  <p>I <i>think</i> that the unique identifier is the <a href="http://msdn.microsoft.com/en-us/library/microsoft.exchange.webservices.data.emailmessage.internetmessageid(v=exchg.80).aspx" rel="noreferrer"><code>EmailMessage.InternetMessageId</code></a>, and that this property uses the <b>PR_ENTRYID</b>, except that I can't find any documentation to support this.</p>
  
  <p>Do all emails have a unique identifier and is that (in Exchange) the InternetMessageId?</p>