🚀 go-pugleaf

 > It doesn't work: Doing a C-h c, then choosing the new menu items,
 > gives me `menu-bar ... is undefined'.  Looking at your source it seems
 > that you indeed have empty menu entries.
This menu items work only if you have a secondary file directory (to
handle folders), thus you must define rmail-secondary-file-directory.

Here is my default (in my .emacs):
(setq rmail-file-name "~/RMAIL/rmail")
(setq rmail-secondary-file-directory "~/RMAIL")
(setq rmail-secondary-file-regexp "^[a-zA-Z]*[-]*[a-zA-Z]*$")

If secondary file directory is not defined, the menu items shouldn't
be visible. This is your case. Try replacing this:
(define-key rmail-mode-map [menu-bar classify input-menu]
  '("Input Rmail File"))

(define-key rmail-mode-map [menu-bar classify output-menu]
  '("Output Rmail File"))

by this:

(define-key rmail-mode-map [menu-bar classify input-menu]
  '(nil))

(define-key rmail-mode-map [menu-bar classify output-menu]
  '(nil))

in rmail.el. Likewise in rmailsum.el.