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Mailbox is a desktop mail client for Linux: Mail, Calendar, People, Tasks, Notes, Journal and Feeds 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.

Pages

• 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.