We typically do not change/modify provided headers, we just add additional headers (other than the standard MTA headers).  



These are the only headers we currently add:
  •  X-Mail-Handler
  •  X-Originating-IP
  •  X-Report-Abuse-To
  •  X-MHO-User
  •  X-MHO-RoutePath
  •  X-AuthUser
If the sender is setting any of these headers in the source message we will overwrite the values.