If you start using any of the mail back ends, they have the annoying habit of assuming that you want to read mail with them. This might not be unreasonable, but it might not be what you want.
If you set mail-sources and
nnmail-spool-file to nil, none of the
back ends will ever attempt to read incoming mail, which should
help.
This might be
too much, if, for instance, you are reading mail quite happily
with nnml and just want to peek at some old
(pre-Emacs 23) Rmail file you have stashed away with
nnbabyl. All back ends have variables called
back-end-get-new-mail. If you want to disable the
nnbabyl mail reading, you edit the virtual server
for the group to have a setting where
nnbabyl-get-new-mail to nil.
All the mail back ends will call
nn*-prepare-save-mail-hook narrowed to
the article to be saved before saving it when reading incoming
mail.