How do I modify a user's email display name in Outlook to read "Consultant - John Smith" but have that not appear outside our corporate network (i.e. read "John Smith")?

I have been asked to modify display names for consultants so that our users can more easily recognize the fact that outside vendors are on the email chain.  The challenge I am facing is limiting this to only be visible within our corporate network.  

The basic idea would be to have the display name read "Consultant - John Smith".  However, this carries over to emails sent outside our domain (i.e. I can see it in my Gmail as well as my corporate email).

I know that I can make my mail relay strip this out, but that is a lot of overhead that I do not want to place on that device.  I want Exchange or Outlook to do that instead.