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Mailbox is a desktop mail client for Linux: Mail, Calendar, People, Tasks, Notes, Journal, Feeds and Weather in one application, over open protocols only — IMAP, POP3, SMTP, CalDAV, CardDAV — with everything stored locally and no service behind it.

The README carries the full feature set, the build instructions and the packaging story. Releases ship a self-contained tarball and a .deb, with an AUR PKGBUILD in the tree — latest release.

The modules

One page each: what the module is for, the views it offers, and the handful of things people actually go looking for.

• Mail (Ctrl+1) — the three views, the search grammar, rules that keep running while your machine is off, junk, Undo Send, and sending as more than one address.
• Calendar (Ctrl+2) — the five arrangements, sweeping empty time into an appointment of the length you want, the arrow keys, invitations, and printing.
• People (Ctrl+3) — the five views, the alphabet index, Select Names, contact groups, linking duplicates, and what Activities can and cannot find.
• Tasks (Ctrl+4) — the three views, the arrangement gallery, flagged mail as to-dos, and recurring chores.
• Notes (Ctrl+5) — a pad, and deliberately little else.
• Journal (Ctrl+8) — the timeline, its six views, and the two things it does not do.
• Feeds (Ctrl+9) — the RSS reader: boards, Read Later, mute filters, and reading the publisher's page in place.
• Weather (Ctrl+0) — forecasts for the places you keep, found by city or zip code: warnings and the forecasters' discussion in the US, air quality, a map played through time, and the next three days on the calendar.

Reference

• Keyboard shortcuts — every shortcut the application ships with, by window and by module, and the keys each surface answers itself: the arrows through a calendar grid, Tab through a day's appointments, the ends of a list. Generated from the command catalogue, so it is what the application does rather than what somebody remembered.
• Themes — the theme engine in short: backgrounds, palettes, the editor, the importer and the in-app theme browser — and the plain words about community themes: what they are, what Mailbox does and does not do with them, their licences, and exactly what touches the network (only the browser, only while it is open).
• Plugins — writing one: the manifest, the IPlugin entry point, and every surface the host offers — commands, a ribbon tab of your own, columns on the message list, bars above a message, the mail and PIM records, the arrival and sending hooks — with the permission each one is gated by. Includes the two things to read first: what installing a plugin actually trusts, and what the GPL asks of one you distribute.
• Hardening — how the application defends itself, layer by layer: the sanitizer in front of the rendering engine, the engine's own sandbox, the systemd unit the launcher confines everything in, credentials, what leaves the machine, and the supply chain. Every claim points at the code at the current release tag, or at a check you can run yourself.