Mail Spool
..........

   The "nnml" spool mail format isn't compatible with any other known
format.  It should be used with some caution.

   If you use this backend, Gnus will split all incoming mail into
files; one file for each mail, and put the articles into the correct
directories under the directory specified by the `nnml-directory'
variable.  The default value is `~/Mail/'.

   You do not have to create any directories beforehand; Gnus will take
care of all that.

   If you have a strict limit as to how many files you are allowed to
store in your account, you should not use this backend.  As each mail
gets its own file, you might very well occupy thousands of inodes
within a few weeks.  If this is no problem for you, and it isn't a
problem for you having your friendly systems administrator walking
around, madly, shouting "Who is eating all my inodes?! Who? Who!?!",
then you should know that this is probably the fastest format to use.
You do not have to trudge through a big mbox file just to read your new
mail.

   `nnml' is probably the slowest backend when it comes to article
splitting.  It has to create lots of files, and it also generates NOV
databases for the incoming mails.  This makes is the fastest backend
when it comes to reading mail.

   Virtual server settings:

`nnml-directory'
     All `nnml' directories will be placed under this directory.

`nnml-active-file'
     The active file for the `nnml' server.

`nnml-newsgroups-file'
     The `nnml' group descriptions file.  *Note Newsgroups File
     Format::.

`nnml-get-new-mail'
     If non-`nil', `nnml' will read incoming mail.

`nnml-nov-is-evil'
     If non-`nil', this backend will ignore any NOV files.

`nnml-nov-file-name'
     The name of the NOV files.  The default is `.overview'.

`nnml-prepare-save-mail-hook'
     Hook run narrowed to an article before saving.

   If your `nnml' groups and NOV files get totally out of whack, you
can do a complete update by typing `M-x nnml-generate-nov-databases'.
This command will trawl through the entire `nnml' hierarchy, looking at
each and every article, so it might take a while to complete.