Re: Recognize mbox files?

[Richard M Stallman]

    A better idea would be for Rmail to detect whether it has already
    munged the file (a quick grep for "^X-RMAIL-.*" headers should do) and
    if not make the buffer read-only.

That sounds like a good idea.
It should be enough to check the first message for that header,
since a previous visit with Rmail will necessarily have displayed the
first message and put that header into it.