Which backend to use with procmail?

[XeCycle]

Hello, I'm a convert from mutt, and is new to Gnus.  I have a typical
mutt configuration: getmail -> procmail -> mutt -> msmtp.  `getmail' is
used to fetch mails from the POP server, and `procmail' to deliver (or
split them, in the Gnus words) them into different maildirs, and of
course `msmtp' to send them.

Now I still want to use `procmail' to do the splitting, as I have the
existing configuration there and it would be fast to do the splitting
outside Emacs.  But I'm confused by what I've got.  Now I have these in
my .gnus.el for mail:

> (setq gnus-select-method '(nnmaildir "Mailbox"
>                                    (directory "~/Mail")
>                                    (get-new-mail t)
>                                    (expire-age never)))

I haven't set the `mail-sources' variable.  Now this does not work as I
expected.  What should I do?  My ~/Mail consists of folders that are
maildirs.

Thank you.