To clarify, Notmuch is a mail indexing and search system with a command-line/library API. It's very easy to integrate with mail frontends because of good design decisions (eg. it does very little, everything can spit out JSON for easy parsing, ...). As a result, many excellent frontend clients were written on top of it (an emacs mode that I use, a Mutt/Sup-like curses client, a GTK one, ...)
Notmuch does two things:
- Indexes a Maildir and places messages into a Xapian fulltext database.
- Provides an API for tagging, threading, retrieving, searching, creating template replies, etc. for the messages in its index.
Getting mail into the folder is the burden of another program (say offlineimap or getmail), and sending mail is also handled by other tools (say MSMTP or sendmail).
I think Notmuch would have made an absolutely killer framework to base Mailpile off of.
