If you've been looking for a fast, global-search and tag-based email system to use within your text editor or in a terminal...

If you're the kind of person that gets excited about being able to write shell scripts for exploring and manipulating your email...

If you're a developer of an existing email program and would love a good library interface for fast, global search with support for arbitrary tags...

If you want the convenience of fast, reliable search of all your email, but don't want to give a 3rd-party access to your email...

Then notmuch may be exactly what you've been looking for.