Mairix searches local mail—that means, mairix absolutely
must have direct access to your mail folders. If your mail
resides on another server (e.g. an IMAP
server) and you happen to have shell access,
nnmairix supports running mairix remotely, e.g. via
ssh.
Additionally, nnmairix only supports the
following Gnus back ends: nnml,
nnmaildir, and nnimap. You must use one
of these back ends for using nnmairix. Other back
ends, like nnmbox, nnfolder or
nnmh, won't work.
If you absolutely must use mbox and still want to use
nnmairix, you can set up a local
IMAP server, which you then access via
nnimap. This is a rather massive setup for accessing
some mbox files, so just change to MH or Maildir already...
However, if you're really, really passionate about using mbox,
you might want to look into the package mairix.el, which comes with Emacs 23.