I never used notmuch, but I was much inspired by it when I saw a talk about it at linux.conf.au a few years back. Later I ended up stealing several ideas from

I never used notmuch, but I was much inspired by it when I saw a talk about it at linux.conf.au a few years back. Later I ended up stealing several ideas from notmuch about how to interface with Xapian, like the one-letter prefixes.