As you can see on this screenshot, tinymail‘s default ui components for viewing messages and for viewing mime parts are now aligned correctly for displaying message/rfc822 mime parts (like forwarded messages). They will now for example display attachments inside a forwarded message and they are hidden using a typical GtkExpander. Of course is all the behaviour of the default message view implemented for gtk+ inheritable and overridable.
ps. Yes, It’s still in 1974, I haven’t found the problem yet.