Most terminal email clients assume you want a mail reader. a2-stuff’s Indicium Mail TUI assumes you want a mail reader that can also hand your inbox to an AI agent and step out of the way. That second part is what makes it worth installing.
The app is a single Rust binary (a program compiled to one executable with no runtime dependencies) that talks IMAP (Internet Message Access Protocol, the standard “read mail from a server” protocol) and SMTP (Simple Mail Transfer Protocol, the standard “send mail to a server” protocol) against any provider, caches messages in SQLite (a small embedded SQL database stored in a single file), and renders everything in a three-pane layout: account list on the left, message list in the middle, reading pane on the right. None of that is unusual. What is unusual is that the same binary can spawn a local Claude, Gemini, or Codex CLI and use that to draft replies on your behalf. It can also expose its own mail-management tools over MCP (Model Context Protocol, an emerging standard for letting AI agents call external tools through a JSON-RPC interface), so a separate agent elsewhere on your machine can ask it to fetch the latest message from a folder, mark it read, or compose a response.
In practice, that means you can keep your normal keyboard-driven email workflow, and reach for AI only when you want it. The agent does not need its own mail credentials, because it talks to Indicium, not to your IMAP server directly.
The local-first pitch
The selling point is not the terminal interface. Plenty of MUAs (mail user agents, the technical name for email clients) do TUI (text-based user interface) email well: NeoMutt, aerc, meli, himalaya, sup, the alpine family. What those clients do not give you is a structured surface an AI can drive.
Indicium exposes both directions of that surface. In one direction, an optional AI integration runs locally on your machine: install Claude Code or the Gemini CLI or Codex, point Indicium at it, and the compose window gets an “AI draft” affordance. In the other direction, Indicium runs an MCP server (a small process that listens on a port for agent calls) that lets any compatible agent call back into your inbox. So you could be working in a separate coding agent and say, “summarize the latest three messages from my CI alias,” and that agent would query Indicium, get the message contents, and respond.
The local-first framing matters because cloud-based “AI for your email” usually means your inbox is being shipped to a third party. Indicium’s draft helper runs against a CLI on your machine, which means the message text never leaves your laptop. The MCP server side runs locally too.
What you get for free with the standard install
The baseline is a competent modern MUA. Indicium supports implicit TLS (Transport Layer Security, the encrypted variant of IMAP/SMTP where the connection is encrypted from the first byte), STARTTLS (the variant where the connection starts unencrypted and upgrades to encrypted mid-handshake), and unencrypted connections, so it will talk to almost any provider. OAuth2 PKCE (Proof Key for Code Exchange, the modern OAuth flow designed for native apps without a server secret) is built in for Gmail, Microsoft 365, and Yahoo, with a custom-provider path for self-hosted setups. Multi-account is first-class; switching between work and personal inboxes is a sidebar tap.
The local cache is the part that makes the interface feel fast. Messages live in SQLite using WAL mode (write-ahead logging, which lets readers and a writer operate on the database without blocking each other), and full-text search runs against SQLite FTS5 (SQLite’s built-in full-text search engine, version 5). For a personal mailbox with tens of thousands of messages, search is sub-second. Attachments are previewed inline: images render using truecolor half-block graphics (a terminal trick that uses background colors to draw pixel-accurate images using two cells per pixel), and PDFs, Word docs, source files, and Markdown all open in the relevant viewer without leaving the app.
RFC 2177 IDLE is the other quiet win. IDLE is the IMAP extension that lets the server push new mail to a connected client instead of forcing the client to poll. With IDLE enabled, new messages show up in the message list the instant the server delivers them, with no manual refresh. For people who treat the inbox as a live feed, this is the difference between a TUI mail client and a TUI mail client that actually behaves like a modern one.
A practical baseline of what the install covers:
- IMAP and SMTP read and send over implicit TLS, STARTTLS, or unencrypted connections
- OAuth2 PKCE for Gmail, Microsoft 365, Yahoo, and any custom IMAP provider you point it at
- Multi-account support with a sidebar for switching between inboxes
- Local SQLite cache in WAL mode with FTS5 full-text search across all messages
- Compose, reply, reply-all, forward, plus attachment viewer for images, PDFs, Word, Markdown, and source
- Inline image preview using truecolor half-block terminal graphics
- RFC 2177 IDLE for server-pushed new mail with no polling
- Configurable themes, resizable panes, and mouse support throughout
The architecture under the hood
The interesting design choice is that the same binary serves three roles: TUI client, local AI integration host, and MCP server. There is no separate daemon process to manage. The compose window opens, calls out to a configured CLI when you ask for an AI draft, and renders the result inline. The MCP server only spins up when an external agent connects; you can leave it off entirely if you do not need it.
For local AI drafting, the integration is intentionally narrow. Indicium does not try to be a chat client. It hands the message body and the surrounding thread context to the CLI you point it at, and renders whatever that CLI writes back. This means you can swap in whatever model you trust. If you want Claude Code to draft replies, you can. If you prefer the local Gemini CLI, you can. If you want Codex because you live in a coding workflow already, you can. The CLI does the work, and Indicium does the email plumbing.
For MCP exposure, the surface is similarly small. The server speaks standard MCP over stdio (standard input/output, the simplest transport for local processes) by default, with a config flag to bind to a TCP port if you want a separate agent on another host to drive it. The tools it exposes are the obvious ones: fetch a message, list a folder, mark read, compose and send. Nothing exotic, but enough that an agent can pull a message, summarize it, draft a reply, and hand the reply back to you for review before anything leaves your outbox.
Trade-offs
The local-first AI integration is not magic. You still need to install and configure a CLI on your own machine, and the quality of the AI draft is exactly the quality of the model and prompt behind that CLI. Indicium is not in the loop on prompt design, so a draft that ignores your house style is a prompt problem, not an Indicium problem. If you do not already use a local coding agent, the MCP server side of this app is also overkill for you; a regular mail client would do.
MCP as a protocol is also still moving. Server implementations differ on edge cases, and Indicium’s MCP surface is narrower than what a custom IMAP-scripting workflow could give you. If you need to script complex folder transforms or auto-label rules, you are better off using himalaya or aerc with a sidecar script. Indicium’s automation story is “let an AI agent query me,” not “let me run arbitrary logic.”
Finally, the install footprint is small but real. The Rust binary compiles to a single static file with no runtime dependencies, which is a major plus over Mutt’s patch-pile or alpine’s ncurses dependency, but it is still one more thing you have to keep updated. If you prefer a packaged distribution, you will be waiting for your distro (your Linux distribution, the operating system variant you installed, like Ubuntu or Fedora) to pick it up.
When this is the right tool
Indicium Mail TUI is the right choice if you live in the terminal, want a fast cached IMAP client with proper modern auth, and you also want a structured way for a local AI agent to read and write your mail without going through a third-party proxy. If you only need the first half, himalaya or aerc gets you there with less surface area. If you only need the second half, a cloud-based assistant will be simpler.
For the specific case of “I want my coding agent to be able to summarize or compose mail without giving it my IMAP password,” Indicium is one of the few tools that solves that out of the box. That is the install that earns its keep.