title: Tag Outlook MailItem with ID number before send without causing TNEF (RTF) send
body: <p>I have an Outlook add-in which assigns a UserProperty to a MailItem before it is sent:</p>
  
  <pre><code>Outlook.UserProperty prop = mail.UserProperties.Add("XXXX", Outlook.OlUserPropertyType.olText);
  prop.Value = "YYYY";
  </code></pre>
  
  <p>It is known (see <a href="https://stackoverflow.com/questions/16575805/stop-outlook-from-converting-html-to-rtf">Stop Outlook from converting HTML to RTF</a> for example) that doing this causes the email to be sent using TNEF (ie RTF format, the dreaded winmail.dat).</p>
  
  <p>My question is, is it safe to simply un-set the TNEF property? The following code will do that:</p>
  
  <pre><code>mail.PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/id/{00062008-0000-0000-C000-000000000046}/8582000B", false);
  </code></pre>
  
  <p>The issue here is that I have to do this on <code>ItemSend</code> - I can't do it straight after I set the property, because it's always <code>false</code> anyway at that point. It only turns <code>true</code> when I hit Send.</p>
  
  <p>I don't mind the fact that un-setting the TNEF property will mean that the property does not get sent with the email. I am more concerned that there may be other situations where TNEF is actually required and that this code would break them.</p>
  
  <p>Or alternatively, is there a better way altogether of "tagging" an email with a custom ID number before it is sent?</p>