Unfortunately, this is outside the scope of Rebex Secure Mail.
It's also outside the scope of Internet mail standards. The message body is just a text or HTML document with no prescribed structure, and different mail agents use different approaches to construct their replies. Some mail agents (Gmail is a prime example) employ complex heuristics to determine which parts of the message body are the reply and which belong to previous messages in the thread, but this is not a part of Rebex Secure Mail feature set. Sorry!