Rmail Babyl
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The "nnbabyl" back end will use a babyl mail box (aka. "rmail mbox")
to store mail. `nnbabyl' will add extra headers to each mail article
to say which group it belongs in.
Virtual server settings:
`nnbabyl-mbox-file'
The name of the rmail mbox file.
`nnbabyl-active-file'
The name of the active file for the rmail box.
`nnbabyl-get-new-mail'
If non-`nil', `nnbabyl' will read incoming mail.