title: Exchange - Removing word "EXTERNAL" from Subject line when replying to an external email
body: <p>We have Exchange server 2013. I am testing a security method to warn users when external emails are received. This is I am doing by putting "EXTERNAL" text in front of subject-line of incoming emails except if the email-subject already has the text. This is working fine.</p>
  
  <p>Now, what I am trying to do is to remove the text "EXTERNAL" when user will reply to the email. I am having a hard time getting a way to achieve it. Exchange rules, so far I have looked into, does not have anything to remove from subject-line, only to prepend.</p>
  
  <p>How can I do it? My goal is to flag incoming external emails and show it users Outlook client in a way so users would see. I want to use Exchange server built-in functionalities, or from Outlook if the settings can be managed by group-policy and users wont be able to change it.
  Thanks and appreciate your help.</p>