9.4.2 nnmairix requirements

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.