Editing an SMTP header with an Exchange 2007 Transport Agent

I'm currently writing an Exchange 2007 Transport Agent to replace some headers in all outgoing mails from a particular sender. I managed to replace the 'From' SMTP header successfully, but rewriting the 'Return-Path' header does not seem to work.

To make this all happen, I have written a custom SmtpReceiveAgent and subscribe to the OnEndOfData event like this:

private static void MyAgent_OnEndOfData(ReceiveMessageEventSource source, EndOfDataEventArgs e)
        {

            try
            {
                var address = e.MailItem.Message.From.SmtpAddress;
                if (address.ToLower().EndsWith("[internal email domain]"))
                {
                    // replace the From: header - WORKING FINE!
                    e.MailItem.Message.From = new EmailRecipient("[displayname]",
                                                                 "[email address]");

                    // replace the Return-Path: header - NOT WORKING!
                    var headerList = e.MailItem.Message.RootPart.Headers;
                    var header = (AddressHeader)headerList.FindFirst("Return-Path");
                    var newheader = new AddressHeader("Return-Path") { Value = "[email address" };
                    headerList.ReplaceChild(newheader, header);
                }
            }
            catch (Exception ex)
            {
               // do something useful here
            }

        }

Per the RFCs, the Return-Path header is supposed to be set by the recipient's SMTP server. If a Return-Path header exists in the email, it is to be removed, and reset by the recipient's server.

Maybe what you are seeing, is the correct implementation of the RFCs.