Variable: rmail-insert-mime-forwarded-message-function

Function to insert a message in MIME format so it can be forwarded.
This function is called if `rmail-enable-mime' and
`rmail-enable-mime-composing' are non-nil.
It is called with one argument FORWARD-BUFFER, which is a
buffer containing the message to forward. The current buffer
is the outgoing mail buffer.