indexes and searches locally-stored email

 mairix is a program for indexing and searching email messages stored
 offline on your computer. mairix supports the Maildir, MH folders,
 and mbox formats.
 .
 Some features:
 .
  * Indexing is fast. It runs incrementally on new messages - any
    particular message only gets scanned once in the lifetime of the
    index file.
 .
  * Searching is fast.
 .
  * The search mode populates a "virtual" maildir folder with symlinks
    which point to the real messages. This folder can be opened as usual
    in your mail program.
 .
  * Indexing and searching is based on words. The index file tabulates
    which words occur in which parts (particular headers + body) of which
    messages.