Updated May 1, 2026

A native client for the protocol Fastmail invented. Plus your other accounts.

Fastmail is the best independent email service we know of. They wrote JMAP, the modern protocol that replaces IMAP, and they run mailboxes that are quietly faster, cleaner, and more thoughtful than the rest of the industry. This page is for the people who already love Fastmail and want a native desktop and mobile client for it, particularly if Fastmail is one of several mailboxes they answer to.

Fastmail is the service. Epistles is a client.

We are not a Fastmail competitor. We do not run mail servers, host calendars, or sell domains. Fastmail does all of that, and does it well, and the right recommendation to anyone choosing a mail provider in 2026 is still to choose Fastmail. Nothing on this page is meant to suggest otherwise.

What Epistles is, is the client we wished we had for our Fastmail accounts. A native macOS application, a native Linux application, native iOS and Android, all speaking JMAP to Fastmail directly, and quietly hosting your other mailboxes alongside in their own native protocols.

JMAP, the protocol Fastmail invented

JMAP is a JSON-based email protocol designed as the modern successor to IMAP. It is documented as RFC 8620 (core) and RFC 8621 (mail), authored largely by Fastmail engineers, with Fastmail running the first production server. By the protocol's own measurements, JMAP uses roughly two to three times less power than IMAP on real devices, transfers a fraction of the bytes (kilobytes instead of megabytes for a full mailbox resync), and has a specification five times shorter than IMAP's.

More importantly, JMAP speaks the language of how email actually works in 2026. Threads are a first-class concept, not something the client reconstructs from References headers. Labels are a first-class concept, not folders pretending to be tags. Push is built into the protocol, not bolted on via IDLE. Server-side filters and identities are exposed and editable. None of that round-trips cleanly through IMAP.

And yet most third-party clients, on most platforms, still reach Fastmail through IMAP. Apple Mail does. Thunderbird does. Mailbird does. Outlook does. Each of those clients is fine, and each loses something in the translation: labels collapse into folder paths, push degrades to polling or IMAP IDLE, threading becomes the client's best guess. Epistles speaks JMAP to Fastmail natively. As of this writing, it is one of a small handful of third-party clients that do.

Fastmail's own desktop is a wrapper around the web app

In October 2025 Fastmail shipped a desktop application for Mac, Windows, and Linux. It is, as their own engineering posts and the press coverage that followed have made clear, an Electron wrapper around their progressive web app. It picks up native notifications, dock badging, default-mail-client registration, and offline support, and it is, by every account, a polished piece of work. For someone who reads only Fastmail mail and likes the Fastmail web interface, it is the right answer.

Epistles made a different choice. The macOS app is React Native macOS, compiled native, signed and notarised. The Linux app is Tauri 2, shipped as both a signed .deb and self-hosted APT, DNF, and Flatpak repos. None bundles Chromium. Neither is a wrapper around a web app. We respect Fastmail's decision, the engineering economics of one team supporting one HTML codebase across web, desktop, and mobile are real, and we do not have a Fastmail-sized team. We simply wanted something different for ourselves, and we suspect a few other people did too.

Fastmail is the start, not the end

The reason this page exists, more than anything else, is that the people who love Fastmail rarely have only Fastmail. They have a work Microsoft 365 mailbox they cannot move. A Gmail address that has been their primary identity since 2008. A ProtonMail account for the political work or the legal correspondence. An IMAP host for a domain they have run since college. Fastmail covers the part of the stack they get to choose. The rest of the stack stays where it is.

Epistles holds all of those mailboxes in one window. Your Fastmail mailbox speaks JMAP. Your Microsoft 365 mailbox speaks MS Graph, with native push, categories, and server-side rules intact. Your Gmail mailbox speaks the Gmail API, with labels staying labels. Your ProtonMail account decrypts on device with OpenPGP.js. Your old IMAP host falls back to IMAP. Each provider's adapter loads independently, so a Fastmail-only user never downloads ProtonMail's crypto, and a Gmail-only user never downloads JMAP code.

One settings panel. One keyboard map. One local SQLite cache that searches across every account in milliseconds, offline, on a plane. A single command palette at ⌘K that finds a thread regardless of which mailbox it lives in. The unified surface is the point of the product.

How Epistles compares to other ways of using Fastmail

Fastmail web: JMAP-native (it is the web app behind the protocol), full integration with calendar, contacts, files, and Masked Email, runs anywhere there is a browser. No multi-account story for non-Fastmail mailboxes; you tab between services.
Fastmail desktop app: The same web app, packaged as Electron. JMAP-native. Native dock, badges, notifications, offline. Same Fastmail-only constraint as the web. Higher memory footprint than a real native client.
Apple Mail with Fastmail: IMAP only, not JMAP. Loses Fastmail labels, server-side filters, and push-via-JMAP. Multi-account works, but every account is reduced to the IMAP feature set.
Thunderbird, Mailbird, Outlook with Fastmail: IMAP only. Same trade-offs as Apple Mail; the Fastmail-specific advantages of JMAP are not available.
Epistles with Fastmail: JMAP-native. Threads, labels, push, server-side filters preserved. Multi-account with Gmail, MS Graph, Proton, IMAP alongside. Real native macOS and Linux clients, native iOS and Android. Windows and a web app at app.epistles.com in build.

What you give up

Honest list. Fastmail's web and desktop clients are integrated end-to-end with the rest of the Fastmail product: calendar, contacts, files, Masked Email, domain management, Quick Settings, sieve filter editor, and the various small niceties Fastmail has accreted over twenty years. Epistles supports Fastmail calendar and contacts via JMAP today, alongside CalDAV and CardDAV for everyone else. We also let you edit the JMAP identities Fastmail exposes, so your sending addresses are first-class.

Where Epistles does not yet reach: Masked Email creation and management is a Fastmail-specific surface, accessed today from the Fastmail web app. Masked aliases that already exist work as send-from addresses through JMAP identities; minting new ones still happens at fastmail.com. The same is true of domain and DNS management, the files tab, and the visual sieve filter editor: those are part of being a Fastmail customer, and Epistles does not try to reimplement them. You keep using fastmail.com for those, the same way you would with any other third-party client.

Snooze and Send Later are also not in Epistles today. Fastmail ships both, in web and in their desktop app. Our roadmap has them in 2026, designed to work without storing your tokens on a server waiting for 7am Tuesday. Until they ship, Fastmail's own clients are ahead of us on those two verbs.

Privacy

Mail moves directly between your device and Fastmail over JMAP. Our servers never see the body, the subject, or the recipients of your Fastmail mail. The Cloud Vault that lets you sign into Epistles on a second device holds your Fastmail JMAP session token as opaque ciphertext, encrypted on your device with a key derived from your Epistles password. We hold the bytes; we cannot decrypt them. The security page has the long version.

If you came here looking

If you read mail only at Fastmail, only on a Mac, and the Fastmail desktop app suits you, keep using it. It is a good piece of software made by the team that built the protocol. If you want a real native client, on Linux as well as Mac, on iOS and Android, or you have one or two other mailboxes to look after, Epistles is built for that case.

Fifteen-day trial, no card up front, three accounts on Free afterwards. Request early access, or read the rest of the site. To the Fastmail team: thank you for JMAP, and for keeping independent email viable. The protocol is the foundation we built on.