how to work with nnfolder?

[Easior]

hi, all.

To archive the messages sent, I use

;; Tell gnus which method to use for archives (nnfolder)
(setq gnus-message-archive-method
           '(nnfolder "archive"
                 (nnfolder-directory "~/.emacs.d/gnus/Mail/archive/")
                 (nnfolder-active-file
"~/.emacs.d/gnus/Mail/archive/active")
                 (nnfolder-get-new-mail nil)
                 (nnfolder-inhibit-expiry t)))
;; Tell gnus into which group to store messages
(setq gnus-message-archive-group
      '((if (message-news-p)
            "nnfolder+archive:sent.news"
            "nnfolder+archive:sent.mail")))

However, there is something weird happened. When I send a message by gnus,
I'll find that a file named 'mail' will be created in the directory
"~/.emacs.d/gnus/Mail/archive/sent/". In fact, it is not correct. Gnus
should create a file, e.g. named '1', in the directory
"~/.emacs.d/gnus/Mail/archive/sent/mail". If I
only use the following configuration

;; Tell gnus into which group to store messages
(setq gnus-message-archive-group
      '((if (message-news-p)
            "nnml+POP:sent.news"
            "nnml+POP:sent.mail")))

everything is ok. What's happened?

Thanks in advance.

Cheers,

Easior

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