Update X-Header on an existing item using EWS Managed API

Hello, 

Does anyone know of a way to insert X-Headers into existing Items, or EmailMessages if not possible on the base Item.

I can create an ExtendedPropertyDefinition 

ExtendedPropertyDefinition xHeader 
                    = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.InternetHeaders, 
                                                    "X-TESTHEADER", 
                                                     MapiPropertyType.String);

and use SetExtendedProperty to assign this ExtendedPropertyDefinition along with a value to a **new** MailMessage which I'm going to send. This works.

The problem is that I need to insert X-Headers into **existing** Items. I can call SetExtendedProperty, and Update on and existing item, but the InternetMessageHeaders collection is not updated. 

I'm thinking that this just isn't possible the with EWS Managed API. 

I'm using EWS Managed API 1.2.1, and interfacing with Exchange 2007

Thanks