This is the (new) MH Mail Handling System, an update on the classic MH Mail Handling system, originally by the RAND Corporation. It's a collection of command-line programs that together provide a mail-user agent, e.g. show(1), and repl(1).
mairix is a program for indexing and searching email messages stored in Maildir, MH or mbox folders.
It is very fast at indexing and searching mails, and creates a virtual folder containing the search results.