dead-letter

Your .eml files deserve a second life.

dead-letter converts email exports into clean Markdown with YAML front matter — threads split, signatures stripped, attachments extracted, calendars parsed. One file or ten thousand.

✨ Features

Full-fidelity conversion — HTML sanitization, Gmail/Outlook thread segmentation, inline image handling, and calendar event summaries
CLI — point it at a file or a directory and go
Local web UI — dark command-center interface with drag-and-drop import, watch mode, conversion grade badges, processing history, and per-job diagnostics
Inbox/Cabinet workflow — drop .eml files into an Inbox, let dead-letter organize the Markdown bundles into a Cabinet
Install validation — dead-letter doctor checks your runtime environment
Conversion report — opt-in JSON report with per-file diagnostics, including attachment referenced/retained counts for automation and audit
MCP server — integrate with Claude Desktop, Claude Code, Codex, and other MCP clients
Claude plugin — one-command install in Claude Code or Cowork with four slash commands (/dead-letter:convert, /dead-letter:summarize, /dead-letter:triage, /dead-letter:cabinet)
Python API — from dead_letter import convert and you're off

🧠 Built for LLM Pipelines

Raw .eml files are noisy input for downstream LLM and retrieval pipelines — MIME headers, multipart boundaries, duplicated HTML/plain bodies, and encoded attachments all get mixed into the text path.

dead-letter normalizes that into Markdown with YAML front matter, so message text and metadata are ready for chunking or indexing without MIME parsing or base64 cleanup. Default convert() and convert_dir() runs write a single .md per message and keep attachment names in front matter.

If you want the filesystem artifacts separated too, bundle and Cabinet workflows write message.md plus retained decoded files under attachments/. The Markdown is ready for text ingestion, while PDFs, spreadsheets, calendar files, and other retained binary attachments stay cleanly split out for whatever downstream parser you already use.

For direct LLM integration, the MCP server lets Claude Desktop, Claude Code, Codex, and other MCP clients call dead-letter's conversion tools without shelling out.

📊 Token-cost benchmarks

dead-letter's value isn't fewer tokens than every alternative — it's fidelity per token: the cheapest representation that keeps the email intact. Measured across a synthetic corpus of HTML threads, attachments, and newsletters (tokenizer o200k_base, medians):

~88% fewer tokens than the raw .eml — a single email with a PDF attachment is ~126k tokens raw vs ~180 converted.
The only representation that keeps the email whole — thread structure, per-message sender attribution, links, and attachment metadata all survive. Naive text extraction is cheaper precisely because it drops them (0/2 attachments retained vs dead-letter's 2/2).

The benchmark is honest about where it loses: naive extraction is fewer tokens when you don't mind throwing away attachments, links, and thread structure. Full method, the complete table (including those rows), tokenizer disclosure, and a one-command reproduce are in benchmarks/.

📦 Install

With Homebrew on Apple silicon macOS:

bash

brew tap BigCactusLabs/tap
brew install dead-letter

The Homebrew formula installs the core CLI only: dead-letter convert and dead-letter doctor. It intentionally does not bundle the optional web UI or MCP server dependency stacks.

With pip:

Terminal

pip install dead-letter            # core + CLI
pip install dead-letter[cli]       # + watchfiles (used by backend/UI watch mode)
pip install dead-letter[ui]        # + web UI, API server, and watch mode
pip install dead-letter[mcp]       # + MCP server

Use pipx for isolated UI or MCP installs:

bash

pipx install 'dead-letter[ui]'    # installs dead-letter and dead-letter-ui
pipx install 'dead-letter[mcp]'   # installs dead-letter and dead-letter-mcp

From source:

bash

git clone https://github.com/BigCactusLabs/dead-letter.git
cd dead-letter
uv sync --extra dev     # all extras
uv sync --extra ui      # UI only
uv sync --extra mcp     # MCP only

🚀 Quick Start

CLI — convert a single file:

bash

dead-letter convert message.eml

Convert a whole directory:

bash

dead-letter convert inbox/ --output out/

Generate a JSON conversion report alongside the output:

bash

dead-letter convert inbox/ --output out/ --report

With --output, the report is written to that output directory as .dead-letter-report.json. Without --output, file conversions write the report next to the source message and directory conversions write it to the input directory root.

Check your runtime environment:

bash

dead-letter doctor

Directory conversion scans recursively for .eml files, matches the suffix case-insensitively, skips symlinked files whose resolved targets escape the requested input tree, and deduplicates in-tree symlink aliases that resolve to the same message file.

Web UI — start the local server:

bash

dead-letter-ui --host 127.0.0.1 --port 8765

Open http://127.0.0.1:8765 — on first launch, a setup prompt suggests default Inbox and Cabinet folders. Configure or skip to start converting. Import .eml files with drag and drop or the file picker. Single-file imports use file mode, while multi-file drops create one directory-mode batch job. Mixed drops ask for confirmation before skipping non-.eml files. The backend enforces a 100 MB per-file import limit for both single and batch uploads.

From a source checkout, prefix with uv run:

bash

uv run dead-letter convert message.eml
uv run --extra ui dead-letter-ui --host 127.0.0.1 --port 8765

🐍 Python API

server.ts

from dead_letter import convert

result = convert("message.eml")
print(result.subject, result.sender)
print(result.output)  # path to the generated .md

With options:

server.ts

from dead_letter import convert, ConvertOptions

result = convert("message.eml", options=ConvertOptions(
    strip_signatures=True,
    strip_quoted_headers=True,
))

Strip signature images (logos, social icons) and tracking pixels:

python

result = convert("message.eml", options=ConvertOptions(
    strip_signature_images=True,
    strip_tracking_pixels=True,
))

When enabled, these filters remove matched images from rendered Markdown and omit stripped inline signature/tracking assets from bundle attachment output.

Bundle conversion (Markdown + attachments + source in one directory):

server.ts

from dead_letter import convert_to_bundle

bundle = convert_to_bundle("message.eml", bundle_root="cabinet/", source_handling="copy")
print(bundle.markdown)     # cabinet/message/message.md
print(bundle.attachments)  # retained extracted files under cabinet/message/attachments/

source_handling="copy" preserves the original .eml in place. If omitted, convert_to_bundle() defaults to source_handling="move" and moves the source message into the bundle.

Retained extracted attachment filenames are normalized to safe basenames before they are written under attachments/.

Quality diagnostics include referenced/retained attachment counts when a message has attachments eligible for retention, so dropped artifacts are machine-detectable. See Quality Diagnostics.

Batch:

server.ts

from dead_letter import convert_dir

for r in convert_dir("inbox/", output="out/"):
    print(f"{'✓' if r.success else '✗'} {r.source.name}")

🔌 MCP Server

dead-letter ships an MCP server so LLM clients can convert .eml files directly without shelling out.

Install and launch:

Terminal

pip install dead-letter[mcp]
dead-letter-mcp

From a source checkout:

bash

uv run --extra mcp dead-letter-mcp

Claude Desktop — add to claude_desktop_config.json:

config.json

{
  "mcpServers": {
    "dead-letter": {
      "command": "uv",
      "args": ["--directory", "/path/to/dead-letter", "run", "--extra", "mcp", "dead-letter-mcp"]
    }
  }
}

Claude Code or Cowork (recommended — Claude plugin):

Code

/plugin marketplace add BigCactusLabs/bigcactuslabs-plugins
/plugin install dead-letter

The plugin bundles the MCP server (via uvx, no pip install needed — just uv on PATH) and adds four slash commands: /dead-letter:convert, /dead-letter:summarize, /dead-letter:triage, /dead-letter:cabinet. Email content handled through the plugin is treated as untrusted data, not instructions, so tool-use, credential, and exfiltration requests embedded in messages are not followed. Source under plugin/.

The marketplace pins each published plugin tag and commit. Release automation updates that pointer only after the bundled MCP server's exact PyPI version is live, so Claude Code and Cowork resolve the same reproducible release.

Claude Code (manual MCP add — alternative):

Terminal

claude mcp add dead-letter -- uv run --extra mcp dead-letter-mcp

Codex:

bash

codex mcp add dead-letter -- uv run --extra mcp dead-letter-mcp
codex mcp list

The codex mcp add command registers the local dead-letter MCP server, and codex mcp list verifies that it's available.

Tools

Tool	Required arguments	Returns
`convert_eml`	`eml_path`	Markdown text. Also writes a file when `output_path` is given.
`convert_eml_to_bundle`	`eml_path`, `bundle_root`	JSON with `bundle_path`, `markdown_path`, `attachment_paths`. Copy-only: the original `.eml` is never moved or deleted.
`convert_directory`	`directory`, `output_directory`	JSON summary. Capped at 50 `.eml` files per call.
`get_diagnostics`	`eml_path`	Quality and structure JSON. Writes nothing permanent.

All four take a preset (default, clean, verbose, raw) and per-flag overrides. Full contract, including the MCP-only constraints and the error-text table: docs/reference/v4-runtime-contracts.md.

🗂 Project Structure

Code

src/dead_letter/
├── core/           # conversion pipeline (MIME, HTML, threads, rendering)
├── backend/        # CLI, API server, job runner, watch mode, MCP server
└── frontend/       # static web UI (Alpine.js ES modules + vanilla fetch)
tests/
├── core/           # conversion pipeline tests with .eml fixtures
├── backend/        # API, job, and watch tests
├── plugin/         # Claude plugin manifest, skill, and command tests
└── frontend/       # JS unit tests

🧪 Testing

bash

uv run pytest -q tests/core        # conversion pipeline
uv run pytest -q tests/backend     # API and job runner
uv run pytest -q tests/plugin      # Claude plugin manifest, skill, and command surfaces
node --test tests/frontend/*.test.js     # frontend

CI runs all four on PRs and on pushes to main or feat/** branches with the same commands, plus npx --yes @anthropic-ai/claude-code@2.1.145 plugin validate plugin/ and node --check src/dead_letter/frontend/static/app.js.

📚 Docs

Docs Index — public docs landing page
Runtime Contracts — full API and core behavior spec
Frontend State Model
Quality Diagnostics
Brand & Style Guide
Changelog
Contributing
Agent Guide — operational guide for AI coding agents working in this repo

🔧 Tools We Love

MarkEdit — TextEdit for Markdown, native macOS, ~4 MB. Opens dead-letter output like it was always meant to live there.
mo — local Markdown viewer that renders files in the browser with live reload. Point it at your Cabinet and read converted mail like a feed.

⚠️ Known Limitations

Local-only — no remote server, no auth
In-memory job registry (state resets on restart)
Single-user, single-machine

License

PolyForm Noncommercial 1.0.0 — free for personal, educational, and nonprofit use. Commercial use requires a separate license from Big Cactus Labs.