Why you want to seperate the body and header? Will you add some ideas how this is done? When it is planned? Because for MemoryHole support ['encrypted headers'] (T742) we need to change the logic how the mail is parsend and displayed. The behavior we need:

1. parse the mail with MimeTreeParser and get a messagepartTree back, this will update the header taking into account encrypted headers and update the headers.
2. render the header
3. render the mail

currently we do parsing the mail and rendering the body in one step, so the displayed header are not updated.