[exim] How to get mail "delivered" to a file (in mbox format…

I use gentoo. I emerged exim and fetchmail. Fetchmail evidently gets
mail to my satisfaction, and hands it off to exim. I say this, because
/var/spool/exim/input is stuffed full of messages, courtesy of exim.

I once read email with emacs (M-x rmail), but that nolonger works. The
reason I suppose is that emacs needs mail in mbox format, and although
mail is sitting in /var/spool/exim/input, nevertheless it has not been
"delivered" to the proper place in an acceptable format.

I don't know what the proper place is, but One Step At A Time. First,
I'ld like to get mail "delivered" to some file (I don't care which) in
mbox format. I have stumbled across information that mbox is *not* an
unambiguous format, and what I ought to perfer is "mboxo". In case it
is not part of what exim does, I'ld settle for having mail "delivered"
to a file as a first step, in some format that is not maildir -- emacs
chokes on maildir, or so I've heard. If that is not part of what exim
does, then I'ld like to get mail "delivered" to some file, any file.

I don't know what "delivered" means, but have been given to understand
that it is something I need, and mail sitting in /var/spool/exim/input
is somehow not good enough.

I suppose getting mail "delivered" needs configuration properly setup.
How? Next, some executable needs to be invoked. How?

Advice such as found In Chapter 3, section 13. "Delivery in detail" is
not sufficiently explicit. I need help at a much lower level like for
instance: "the following lines go in the following configuration file"
or "make the following a cron job".

Thanks.