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 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.