mairix is a program for indexing and searching email messages stored in
Maildir, MH or mbox folders.

* Indexing is fast.  It runs incrementally on new messages - any particular
  message only gets scanned once in the lifetime of the index file.

* The search mode populates a "virtual" folder with symlinks(*) which
  point to the real messages.  This folder can be opened as usual in your mail
  program.

* The search mode is very fast.

* Indexing and searching works on the basis of words.  The index file tabulates
  which words occur in which parts (particular headers + body) of which
  messages.

The program is a very useful complement to mail programs like mutt
(http://www.mutt.org/, which supports Maildir, MH and mbox folders) and
Sylpheed (which supports MH folders).

[(*) where the input or output folder is an mbox, a copy of the message is made
instead of symlinking.]