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#### 6.4.5 Mail Back End Variables

These variables are (for the most part) pertinent to all the various mail back ends.

`nnmail-read-incoming-hook`
  The mail back ends all call this hook after reading new mail. You can use this hook to notify any mail watch programs, if you want to.

`nnmail-split-hook`
  Hook run in the buffer where the mail headers of each message is kept just before the splitting based on these headers is done. The hook is free to modify the buffer contents in any way it sees fit—the buffer is discarded after the splitting has been done, and no changes performed in the buffer will show up in any files. `gnus-article-decode-encoded-words` is one likely function to add to this hook.

`nnmail-pre-get-new-mail-hook`
`nnmail-post-get-new-mail-hook`
  These are two useful hooks executed when treating new incoming mail—`nnmail-pre-get-new-mail-hook` (is called just before starting to handle the new mail) and `nnmail-post-get-new-mail-hook` (is called when the mail handling is done). Here's and example of using these two hooks to change the default file modes the new mail files get:

                (add-hook 'nnmail-pre-get-new-mail-hook
                          (lambda () (set-default-file-modes 511)))

                (add-hook 'nnmail-post-get-new-mail-hook
                          (lambda () (set-default-file-modes 551)))

`nnmail-use-long-file-names`
  If non-`nil`, the mail back ends will use long file and directory names. Groups like ‘`mail.misc`’ will end up in directories (assuming use of `nnml` back end) or files (assuming use of `nnfolder` back end) like `mail.misc`. If it is `nil`, the same group will end up in `mail/misc`.
`nnmail-delete-file-function`
  Function called to delete files. It is `delete-file` by default.
`nnmail-cache-accepted-message-ids`
  If non-`nil`, put the `Message-ID`s of articles imported into the back end (via `Gcc`, for instance) into the mail duplication discovery cache. The default is `nil`.
`nnmail-cache-ignore-groups`
  This can be a regular expression or a list of regular expressions. Group names that match any of the regular expressions will never be recorded in the `Message-ID` cache.

  This can be useful, for example, when using Fancy Splitting (see Fancy Mail Splitting) together with the function `nnmail-split-fancy-with-parent`.