KyPost
A native SwiftUI mail client for macOS and iOS. It connects to a KyPost mail relay.
The app talks only to the relay backend. There is no direct IMAP or SMTP. You pair a device one time with a QR code or a deep link. The relay then handles mail access, server-side keyword tabs, push notifications, and contact sync.
Requirements
- macOS 26.0+ or iOS/iPadOS 26.0+.
- Your own reachable KyPost server over HTTPS, with an everyday mailbox account.
- For push: APNs configured on that server.
- Source builds only: Xcode 26 or newer with the matching platform SDKs.
The app uses SwiftData for persistence, URLSession for networking, and WebKit for rendering.
Its one external dependency is GopenPGP — the same OpenPGP implementation Proton uses — consumed as a binary XCFramework through the local package in Dependencies/GopenPGP. That binary is built from source by .github/workflows/gopenpgp-xcframework.yml, from the upstream tag pinned in Dependencies/gopenpgp.env, using upstream's own build.sh plus one linker flag (GODEBUG default asyncpreemptoff=1) that keeps the embedded Go runtime's preemption signal from crashing the app; Package.swift pins its SHA-256, so the bytes cannot change without that line changing. Nothing outside KyPost/Domain/Security/PgpCrypto.swift and its conforming types may reference the library.
Install and pair
As of 2026-10-20, GitHub Releases contains a GopenPGP dependency artifact, not a KyPost
application DMG. The application release workflow exists, but its signed/notarized Mac
DMG and App Store distribution have not been qualified here. Do not mistake the dependency
release for an installable client or assume App Store availability.
For a source installation on a Mac:
- Clone this repository, open
KyPost.xcodeproj in Xcode, and select the KyPost scheme.
- Choose My Mac, or an iPhone/iPad simulator. For a physical iOS device, configure your own Apple signing team/provisioning and the required capabilities; push and shared-group sign-on also need matching server/client configuration.
- Build and run. A simulator can exercise UI but does not qualify Secure Enclave custody, biometric behavior or real APNs delivery.
- In your server’s web frontend, open Settings → Security → Devices → Pair a new device. On a Mac choose Pair this device, or paste the displayed
kypost://native-pair?... link in Settings → Connection → Paste Pairing Link…. On iOS scan the QR. Confirm the server; registration stores per-device credentials and the certificate pin in Keychain.
- Check inbox access and a controlled send/reply through the configured mail service. Configure APNs separately and observe actual delivery before calling push verified.
When application releases are published, install the verified Mac DMG by dragging KyPost to
Applications, or use the actual iOS distribution listing. Updates must retain the same
signing identity and Keychain access groups to preserve access to existing pairing keys.
Until you pair, the inbox says how: on a Mac it gives the web steps above, and on iOS it points to Settings → Connection.
License
MIT, developed by Busnes.app — see LICENSE.txt.
What it can do
- Read mail with folders, keyword tabs and an HTML reader.
- Compose, manage contacts and pair per-device credentials through KyPost.
- Use native macOS windows and shortcuts or the iOS interface.
- Review security controls, platform differences and known gaps in the feature guide.
Documentation
- Development — Identifiers, Architecture, Testing.
- Features — Features, Known gaps.
- Verifying releases — Check the GopenPGP binary against its pin and its build, and what can be checked of the app.
- Assurance case — Threat model, trust boundaries, and why the security requirements are met.
- Strong cryptography — Key-size floors for TLS and OpenPGP, the key inventory, and what is out of scope.
