spark-rs

A personal, Spark-style email client with a Rust core. First target: Windows on ARM (aarch64-pc-windows-msvc).

Created in T3 Code.

Accounts

Settings (Ctrl+,) → Accounts → Add an account. Until the first account is added the app shows a demo mailbox.

Mail is stored in mail.sqlite in the app's local data folder (%LOCALAPPDATA%\dev.sparkrs.desktop on Windows). Passwords and Microsoft sign-ins are kept in Windows Credential Manager under spark-rs, never in files. Removing an account deletes its local mail and saved sign-in; nothing is deleted on the server.

Gmail. Google requires an app password for mail apps: turn on 2-Step Verification, create one at https://myaccount.google.com/apppasswords, then choose Gmail and enter your address and the 16 letters. spark-rs uses imap.gmail.com:993 and smtp.gmail.com:465 and checks the login before saving.

Microsoft 365 (work or school). This needs an app registration in your organization's Microsoft Entra tenant (https://entra.microsoft.com → App registrations → New registration). If you can't register apps, ask IT:

• Supported account types: this organizational directory only.
• Platform: Mobile and desktop applications, redirect URI http://localhost (any port matches; spark-rs picks a free one).
• API permissions, Microsoft Graph, delegated: User.Read, Mail.ReadWrite, Mail.Send, Calendars.ReadWrite, offline_access. If your tenant requires admin consent, IT has to grant it.
• No client secret: desktop apps are public clients.

Then choose Microsoft 365 and enter the Application (client) ID and the Directory (tenant) ID, or your domain such as contoso.onmicrosoft.com. Your browser opens Microsoft's sign-in; when it's done the account appears. Check your company's rules first: using your own client on work mail, and sending work mail to an AI provider for summaries and drafts, may not be allowed.

Other IMAP servers. Choose Other (IMAP) and enter the servers. Ports 993 and 465 use TLS from the start; 143 and 587 use STARTTLS.

What syncs: the inbox (the newest 300 messages on Gmail/IMAP, the last 30 days on Microsoft 365), checked every two minutes and on demand. Scrolling to the end of a list brings in older mail, 100 messages at a time (on Microsoft 365, mail from before the first 30 days doesn't pick up changes made in other apps, such as being read or moved). Done, Delete, read and pinned go back to the server after the 6-second undo window; Done archives (All Mail on Gmail). Snooze, Set Aside, Priority and new-sender screening stay on this computer.

Closing the composer saves the message to the account's Drafts folder (it's also saved as you type), on this computer at once and on the server with the next sync, so it works offline. Opening a draft from Drafts edits it; sending it or Delete Draft removes it from the server too, and undoing the send keeps it. Sent and Drafts list who the mail is to.

Attachments download when you open or save them, also from archived mail. Program files (.exe, .js, .lnk and the like) can only be saved, not opened, and downloaded files are marked as coming from the internet, so Office opens them in Protected View. Attach files with the paperclip, Ctrl+Shift+A, or by dropping them on the window; forwards keep the original's attachments. Messages over 10 MB are stored without their body or file list.

Calendar invitations show as a card in place of the email: the date and time in your time zone, who's invited and how they answered, the call to join, the place and the description. Accept, Maybe or Decline emails your answer to the organizer at once (there's no undo window), and Change lets you pick another. Replies and cancellations show as a banner over the event. The email itself is behind the ⋯ button under the card. Answering doesn't change your own calendar: on Microsoft 365, Outlook keeps showing the meeting as tentative.

Calendar

The Calendar button in the sidebar (it shows today's date) opens your Microsoft 365 calendars in Day, Week or Month view, with the calendars you pick under Calendars. Clicking an event shows its details, a Join button for Teams, Zoom or Meet calls, and Yes / Maybe / No, which answers it in your calendar and tells the organizer. The calendar is read from Microsoft when you open it, every five minutes while it's open, and as you move to other weeks; what was read stays on this computer, so it opens at once. Creating and editing events isn't there yet, and neither are Gmail calendars.

Microsoft 365 accounts added before the calendar arrived signed in without the calendar permission: add Calendars.ReadWrite to the app registration (see above), then choose Sign in on the calendar screen.

A minute before each meeting, a pop-up shows at the top of the screen, over your other windows: who's in it, the time to go (then the time left), and Join, which opens the call. Its other buttons open the day in the calendar, copy the call link, snooze it for a minute and dismiss it; drag it to move it. It stays until the meeting ends unless you join or dismiss it. Only your main Microsoft 365 calendar pops up, without declined, canceled or all-day events, and by default without events that have no one else and no call link, like focus time. To catch meetings added since you last looked, spark-rs reads the next two hours of your calendar every five minutes. Settings → Notifications → Meetings turns the pop-up off and shows a sample.

Notifications and the tray

Closing the window leaves spark-rs running in the notification area, still syncing; click its icon to bring the window back, or right-click it to check for mail or quit. New mail shows a Windows notification with the New Mail sound, and clicking one opens the message. By default only mail from people notifies (Spark's smart notifications); newsletters, notifications, mail from new senders waiting in the Gatekeeper, and an account's first sync stay quiet, as does anything arriving while the window is in front. Settings → Notifications changes this, turns the sound off, stops closing to the tray, or opens spark-rs in the background when you sign in to Windows.

Starting spark-rs again while it runs brings the running window forward.

Sidebar and folders

The sidebar starts as a narrow strip of icons. / or the ≡ button opens it over the list, and Lock Sidebar keeps it open. Right under the Inbox is a top section (Pins, Drafts, Sent, Trash at first), then Folders. Only the top section shows in the narrow strip.

More opens every list you can add: shared lists like Snoozed, Spam, Blocked and Recently Seen, smart folders (saved searches), and each account's own folders and Gmail labels. Clicking one opens it and keeps it under Recents; Ctrl-click or the star adds it to the sidebar. Right-click a sidebar entry to move it to the top, hide its counter, color it, or rename or delete a folder. The + buttons create folders on the server, and the search field's Save button turns a search into a smart folder.

Drag entries in the open sidebar to reorder them or move them between the top section and Folders; Recents can be dragged into either. Drag an email from the list onto a folder (of its own account) to move it there, or onto Inbox, Archive, Trash or Pins; this works on the narrow strip's icons too, and Undo puts it back. With more than one account, the Inbox and the shared lists (Sent, Drafts, Archive, Trash, Spam, Snoozed, Pins…) expand into a row per account that shows just that account's mail.

Moving an email to a folder moves it on the server once the undo window passes. On Gmail, folders are labels: moving between them swaps the labels, and the email keeps its other labels.

A folder shows its newest 200 messages, and 100 more each time you scroll to the end. Folders in the sidebar stay in sync, as does the one that's open; others are fetched when opened.

AI agents

Agents on this computer, such as Claude Code, Codex and Hermes, can work with your mail through spark-cli, which they run as an MCP server (spark-cli mcp). They see nothing until you choose what they may do in each account under Settings → Agents:

• Read: list, search and read mail.
• Triage: also mark mail Done, read, pinned, set aside, snoozed or deleted. Changes reach the server after the usual undo window.
• Send: also send email as you. spark-rs holds each one for 30 seconds and shows it with Undo (or a notification while the window is hidden).

The same page has the command that connects each agent, for example claude mcp add --scope user spark-rs -- "<path>\spark-cli.exe" mcp. spark-cli reaches the running app over a loopback port with a token that only your Windows account can read, and starts spark-rs in the background if it isn't running. From a shell, spark-cli status, spark-cli tools and spark-cli call <tool> '<json>' do the same. Emails are written by other people and can contain text aimed at agents, so allow sending only if you trust the agents you use.

Layout

Building

Everything builds in a Docker image, so the host needs nothing besides Docker. The first run builds the image (a few minutes). On the first Windows build, cargo-xwin downloads the MSVC CRT and Windows SDK (about 5 GB, cached in ~/.cache/spark-rs-cross); using those means accepting Microsoft's license terms for them.

spark-desktop.exe is a single file with the UI embedded; it needs the WebView2 runtime, which Windows 11 includes. On the Windows machine, spark-cli.exe doctor checks that SQLite, TLS and networking work there.

Natively on Windows ARM64

On an ARM64 Windows machine you can skip Docker. You need rustup, Node, Visual Studio Build Tools 2022 with the MSVC ARM64 build tools, and clang-cl: aws-lc-sys won't build with MSVC's cl on ARM64. Either install LLVM (winget install LLVM.LLVM) or add the C++ Clang Compiler for Windows component to the Build Tools. tools\win-arm64\cargo.cmd runs cargo in the ARM64 build environment with clang-cl; it finds cargo in %USERPROFILE%\.cargo\bin and LLVM in its default folder when they aren't on PATH.

From the repo root, in cmd or PowerShell:

The binaries land in target\release\.

For the GreenMail test without Docker, run the standalone jar with Java 17+ (java -Dgreenmail.setup.test.all -Dgreenmail.hostname=127.0.0.1 -Dgreenmail.auth.disabled -jar greenmail-standalone-2.1.5.jar, from Maven Central) and set SPARK_TEST_GREENMAIL=127.0.0.1 before cargo.cmd test.

TLS uses rustls with the aws-lc-rs backend. Don't switch it to ring: ring's build forces plain clang on ARM64 Windows, which fails with cargo-xwin's MSVC-style flags.