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

Features:
* 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.

WWW: http://www.rc0.org.uk/mairix/