mairix is a program for indexing and searching email messages stored
in maildir, MH or mbox folders. Some features are fast and incremental
indexing. The search mode populates a "virtual" maildir (or MH) folder
with symlinks which point to the real messages. The search mode is very
fast, indexing and searching works on the basis of words.

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).