gnus: Mail Folders
6.4.13.10 Mail Folders
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‘nnfolder’ is a back end for storing each mail group in a separate file.
Each file is in the standard Un*x mbox format. ‘nnfolder’ will add
extra headers to keep track of article numbers and arrival dates.
Virtual server settings:
‘nnfolder-directory’
All the ‘nnfolder’ mail boxes will be stored under this directory.
The default is the value of ‘message-directory’ (whose default is
‘~/Mail’)
‘nnfolder-active-file’
The name of the active file. The default is ‘~/Mail/active’.
‘nnfolder-newsgroups-file’
The name of the group descriptions file. 
Newsgroups File
Format. The default is ‘~/Mail/newsgroups’
‘nnfolder-get-new-mail’
If non-‘nil’, ‘nnfolder’ will read incoming mail. The default is
‘t’
‘nnfolder-save-buffer-hook’
Hook run before saving the folders. Note that Emacs does the
normal backup renaming of files even with the ‘nnfolder’ buffers.
If you wish to switch this off, you could say something like the
following in your ‘.emacs’ file:
(defun turn-off-backup ()
(set (make-local-variable 'backup-inhibited) t))
(add-hook 'nnfolder-save-buffer-hook 'turn-off-backup)
‘nnfolder-delete-mail-hook’
Hook run in a buffer narrowed to the message that is to be deleted.
This function can be used to copy the message to somewhere else, or
to extract some information from it before removing it.
‘nnfolder-nov-is-evil’
If non-‘nil’, this back end will ignore any NOV files. The default
is ‘nil’.
‘nnfolder-nov-file-suffix’
The extension for NOV files. The default is ‘.nov’.
‘nnfolder-nov-directory’
The directory where the NOV files should be stored. If ‘nil’,
‘nnfolder-directory’ is used.
If you have lots of ‘nnfolder’-like files you’d like to read with
‘nnfolder’, you can use the ‘M-x nnfolder-generate-active-file’ command
to make ‘nnfolder’ aware of all likely files in ‘nnfolder-directory’.
This only works if you use long file names, though.
Info Catalog
gnus: Article Marks
gnus: Choosing a Mail Back End
gnus: Comparing Mail Back Ends