This is a desktop mail client. It is also a web-mail you can run on a server somewhere if you so choose. It depends on how it is deployed.

Contrasting Mailpile with other tools, one difference is that the basic design is that of a search engine, not a tool for reading mail from folders. Most current desktop mail clients are built on top of a bad paradigm, in my opinion.

Another exciting thing about this model, is that since the UI is a website (of sorts), we can leverage the collective experience and creativity of the web design community. That is a much, much larger pool of talent than UI designers who know C++ or Objective C, or whatever.

Finally, making the app a web server means you get an API to interact with your e-mail almost for free.

I've been working on an extensible console-based mail-client for the past while. (http://lumail.org/)

I abhor the idea that clients only ever interact with messages from a single folder.

notmuch isn't really a client so much as a backend. A better question might be why this isn't using/building upon it.

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.