Open-source Postbox alternatives

Mailspring is a desktop email client for Mac, Windows and Linux, built as a new version of Nylas Mail by one of its original authors. The interface is written in TypeScript with Electron and React, and the mail sync engine is written in C++ and runs locally on your computer. Both parts are open source under GPLv3.

Features include a unified inbox, snooze, send later, mail rules and templates, and a plugin architecture makes the app easy to extend. The README states that these features run in the client and that email credentials are not sent to the cloud. The IMAP-based sync engine replaces the earlier JavaScript sync code used by Nylas Mail.

A paid Mailspring Pro subscription unlocks extra features for people who send a lot of email, such as link tracking, read receipts, mailbox analytics and contact and company profiles. Compiled downloads are offered for Windows, macOS and Linux, including deb, rpm and snap packages.

This repository is an experimental copy of Mozilla's comm-central tree, set up so it can be forked on GitHub. Its README is the Thunderbird project README: Thunderbird is an open-source, customizable email client built on the same Mozilla platform that Firefox uses.

To build Thunderbird you need the Firefox repository as well, because the Thunderbird code lives in a comm subdirectory inside the Firefox source tree and the two share a lot of platform code. The README points to a Getting Started guide, a Building Thunderbird guide, a developer website and rendered source tree documentation.

Contributors can join the project's mailing lists, including a moderated planning list for higher-level topics such as future features, and follow the contributing pages on the Thunderbird site. The code is mostly JavaScript, and this repository holds the source rather than the packaged email application.