The back end nnmairix enables you to call mairix from within Gnus,
either to query mairix with a search term or to update the
database. While visiting a message in the summary buffer, you can use
several pre-defined shortcuts for calling mairix, e.g., to quickly
search for all mails from the sender of the current message or to
display the whole thread associated with the message, even if the
mails are in different folders.
Additionally, you can create permanent nnmairix groups which are bound
to certain mairix searches. This way, you can easily create a group
containing mails from a certain sender, with a certain subject line or
even for one specific thread based on the Message-ID. If you check for
new mail in these folders (e.g., by pressing g or M-g), they
automatically update themselves by calling mairix.
You might ask why you need nnmairix at all, since mairix already
creates the group, populates it with links to the mails so that you can
then access it with Gnus, right? Well, this might work, but often
does not—at least not without problems. Most probably you will get
strange article counts, and sometimes you might see mails which Gnus
claims have already been canceled and are inaccessible. This is due to
the fact that Gnus isn’t really amused when things are happening behind
its back. Another problem can be the mail back end itself, e.g., if you
use mairix with an IMAP server (I had Dovecot complaining
about corrupt index files when mairix changed the contents of the search
group). Using nnmairix should circumvent these problems.
nnmairix is not really a mail back end—it’s actually more like
a wrapper, sitting between a “real” mail back end where mairix stores
the searches and the Gnus front end. You can choose between three
different mail back ends for the mairix folders: nnml,
nnmaildir or nnimap. nnmairix will call the mairix
binary so that the search results are stored in folders named
zz_mairix-<NAME>-<NUMBER> on this mail back end, but it will
present these folders in the Gnus front end only with <NAME>.
You can use an existing mail back end where you already store your mail,
but if you’re uncomfortable with nnmairix creating new mail
groups alongside your other mail, you can also create, e.g., a new
nnmaildir or nnml server exclusively for mairix, but then
make sure those servers do not accidentally receive your new mail
(see nnmairix caveats). A special case exists if you want to use
mairix remotely on an IMAP server with nnimap—here the mairix
folders and your other mail must be on the same nnimap back end.