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.