Notmuch is a free, open-source email search and organization system designed for people who want fast, local access to large mail collections. It indexes messages so users can search across their email and apply flexible tags, helping them find information and keep their inbox manageable. Notmuch is especially suited to use from a terminal or text editor, and its search and tagging capabilities can also be integrated into other email programs through its library interface.

Notmuch is not a complete email service or standalone mail client: it does not receive or send messages, and it has no network or message-composition features. Instead, it focuses on indexing, searching, and tagging stored email, using the Xapian library for search. The project provides command-line tools, an Emacs email client, a Vim client, and an integration script for Mutt, alongside support for other clients and front ends. Notmuch is distributed under the GNU General Public License version 3, with source releases and packages available for a range of operating systems.