rmail-forward-separator-regex

rmail-forward-separator-regex is a variable defined in `undigest.el'.
Its value is

"^----.*\\([Ff]orwarded\\|[Oo]riginal\\).*[Mm]essage"

Documentation:
Regexp to match the string that introduces forwarded messages.
This is not a header, but a string contained in the body of the message.
You may need to customize it for local needs.

You can customize this variable.