Unix Mail Box
.............

   The "nnmbox" backend will use the standard Un*x mbox file to store
mail.  `nnmbox' will add extra headers to each mail article to say
which group it belongs in.

   Virtual server settings:

`nnmbox-mbox-file'
     The name of the mail box in the user's home directory.

`nnmbox-active-file'
     The name of the active file for the mail box.

`nnmbox-get-new-mail'
     If non-`nil', `nnmbox' will read incoming mail and split it into
     groups.