mail-muncher

README

mail-muncher

Give a program its own read-only mailbox, filtered down to exactly the mail it asked for, delivered as files on disk.

mail-muncher pulls messages from a mail provider, evaluates each one against ordered rules, and writes the matches to a directory — byte-faithful .eml, and optionally a markdown rendering with the headers as YAML frontmatter, the body as text, and attachments extracted alongside. A rule can take its filter input from a plain text file that some other program owns, which mail-muncher re-reads at the start of every cycle. That other program changes one line in that file, and the very next cycle delivers different mail — no config edit, no restart, no redeploy.

It reads from any IMAP mailbox — Gmail, Fastmail, iCloud, Proton Bridge, a work account, your own server — or from Gmail's API with a read-only OAuth scope. It runs one-shot for cron, or as a polling daemon, or as a stdio MCP server an agent can query directly. Every mode emits the same machine-readable manifest of what it did, and no mode ever writes to your mailbox.

Two ways to connect a mailbox

Pick one before you install anything. Both are supported, and everything downstream — rules, formats, filenames, the archive layout, the MCP tools — is identical either way.

	provider: imap	provider: gmail
Setup time	~2 min	~10 min in the Google Cloud Console
What you register	nothing	your own Google Cloud project and Desktop-app OAuth client
Credential	an app password from your provider's own settings page	an OAuth token, scope gmail.readonly
How wide that credential is	a full mail credential. An app password can send and delete	read-only, and nothing else
Who enforces read-only	mail-muncher's own code	Google
Expiry	none	every 7 days on a Testing-mode consent screen; mail-muncher auth has to be re-run weekly
Where the secret lives	wherever your password manager already keeps it: password_cmd is run and its stdout is the password. There is deliberately no password key	token.json, mode 0600, written by mail-muncher auth
Which mailboxes	the folders you list in mailboxes:; [INBOX] by default	the whole Gmail account, minus Spam and Trash unless you ask for them
Works with	Gmail, Fastmail, iCloud, Proton Bridge, work accounts, self-hosted	Gmail only
Extra steps	none. There is no auth command on this path	mail-muncher auth, after docs/gmail-setup.md

The ~2 min / ~10 min / 7 days above are the same numbers mail-muncher init and the unconfigured-run guidance print, because they are the numbers that decide this.

The read-only guarantee is real on both paths, but it is not the same guarantee, and flattening the two would be dishonest.

• Gmail: enforced by Google. The only scope requested is gmail.readonly. The token that comes back is incapable of sending, deleting, labelling or modifying — not because mail-muncher declines to, but because Google will refuse the call. A bug in this program cannot reach your mailbox.
• IMAP: enforced by mail-muncher. IMAP has no read-only credential to ask for. An app password is a full mail credential; the protocol will happily let its holder delete a folder. What mail-muncher does instead is refuse to: every folder is opened with EXAMINE and never SELECT, every body is fetched with BODY.PEEK[] and never BODY[] (so mail is never marked read), and there is no code path anywhere in the provider that issues STORE, APPEND or EXPUNGE. Both belts are worn because a server is not obliged to protect a client from itself. That is a strong guarantee and an auditable one — it is just this program's guarantee, not your mail provider's.

If you have no specific reason to want the Gmail API, start with IMAP. It works on a Gmail account too, and it is the path the quickstart takes.

The problem

An automated process needs some mail. A job-search tracker wants replies from companies you applied to. A support bot wants messages from one vendor's domain. A research agent wants every newsletter from three publishers, as text it can actually read.

The usual answers are all bad. Hand the process your inbox credentials and it can read (and send, and delete) everything. Give it a mail API integration and you now maintain an OAuth flow, a sync cursor, MIME parsing, and a dedup story inside every process that wants mail. Or hard-code the filter into a config file, and every change to what it wants is a config edit and a redeploy.

mail-muncher splits that in half. It owns the credentials, the incremental sync, the parsing, and the dedup. The consuming program owns a text file listing what it wants and a directory it reads results from — and, if it prefers to ask rather than watch, a handful of MCP tools over that same directory.

The agent workflow

There are two supported shapes, and they compose. Pick by whether your agent runs on a loop of its own or waits to be asked.

• File drop — mail-muncher runs on a schedule and writes files; the agent reads the directory. Nothing calls anything. This is the shape below.
• Tool call — the agent talks to mail-muncher mcp over MCP and asks questions directly: what am I subscribed to, what arrived, what does this thread say, fetch now. See Shape 2: tool call.

Both read the same archive, and running both at once is normal: a daemon fills the directory while the MCP server answers questions about it.

Shape 1: file drop

The loop is fully decoupled: mail-muncher never calls the agent, and the agent need never call mail-muncher. They share two paths on disk.

1. The agent declares what it wants. Append to a file it owns:

mkdir -p ~/.local/share/agent
cat >> ~/.local/share/agent/domains.txt <<'EOF'
# domains this agent is currently interested in
acme.com
globex.io
EOF

2. mail-muncher subscribes to that declaration. One rule, pointed at the file:

rules:
  - name: agent-inbox
    match:
      from_domains_file: ~/.local/share/agent/domains.txt
    dest: ~/mail/agent-inbox
    formats: [eml, markdown]

3. Every cycle re-reads the file. Run it from cron, or leave the daemon running:

mail-muncher run                 # one cycle — the cron entrypoint
mail-muncher daemon --interval 5m  # poll forever

4. Matched mail lands in dest as files the agent reads.

~/mail/agent-inbox/
└── 2026/
    └── 07/
        ├── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.eml
        ├── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.md
        └── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.attachments/
            └── offer.pdf

The .md is the consumable rendering — parse the frontmatter, feed the body to a model, open the attachments from the sibling directory:

---
subject: 'Re: Your application for Senior Engineer'
from: Jane Doe <jane@acme.com>
from_address: jane@acme.com
from_addresses: [jane@acme.com]
to: [me@example.com]
to_addresses: [me@example.com]
date: 2026-07-28T09:15:00Z
message_id: <abc123@acme.com>
thread_id: 18fe9c0d1a2b3c4d
thread_id_source: provider
in_reply_to: <application-000@example.com>
account: personal
rule: job-search
labels: [INBOX]
attachments: [offer.pdf]
---

Hi there,

Thanks for applying.

## Attachments

- [offer.pdf](1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.attachments/offer.pdf)

thread_id is on every message and is never empty, so grouping a directory into conversations is a sort on one field — no reference chains to reassemble.

5. Optionally, take the manifest instead of walking the tree. --json writes a machine-readable record of the cycle to stdout, one object per account, while every log line goes to stderr:

mail-muncher run --json 2>/dev/null | jq -r '.stored[].path'

Full contract: docs/manifest.md.

Three properties make this safe to put in an autonomous loop:

• Read-only by construction. Nothing in mail-muncher writes to a mailbox. On Gmail that is Google's enforcement of the gmail.readonly scope; on IMAP it is EXAMINE and BODY.PEEK[] and no write path at all. Either way, whatever consumes the output — and whatever bug it has — cannot send, delete, or modify mail. See the comparison above for which of those two guarantees you are getting.
• Idempotent delivery. A message's filename embeds a digest of account + ":" + message id, so its destination path is a pure function of its identity. A file that is already there means "an earlier cycle stored this", and the sink writes nothing. Re-run, replay after losing state, crash mid-cycle, or overlap two cron invocations: the tree converges, and nothing is processed twice.
• Deterministic routing. Rules are ordered and first-match-wins, so each message is written by exactly one rule. Give each consumer its own rule and its own dest, and each gets a private mailbox nothing else writes into.

Delivery is files on disk, and nothing here listens on a network. The contract is the directory, with the manifest as an optional, machine-readable account of what changed.

Shape 2: tool call

mail-muncher mcp is a stdio MCP server over the mail already archived. The agent asks; nothing is scheduled.

{
  "mcpServers": {
    "mail-muncher": {
      "command": "/usr/local/bin/mail-muncher",
      "args": ["mcp", "--config", "/Users/you/.config/mail-muncher/config.yml"]
    }
  }
}

Five tools:

Tool	What it answers
list_rules	What am I collecting, and which senders am I subscribed to right now? Each from_domains_file is re-read on every call.
list_messages	What has arrived? Filter by rule, account, thread or date; optionally grouped into conversations.
search_messages	Where is the message that mentions X? Substring search over subject, sender, recipients, labels, attachment names and body.
read_message	One message in full — metadata, body, attachment names and sizes — and optionally its whole thread in order.
sync	Fetch new mail once, returning the same manifest run --json writes.

It is read-only over mail: no tool sends, deletes, or modifies anything, and sync — the only tool that changes anything at all — can only add files. Filesystem access is jailed to the configured rule dest roots, so the config, any stored credential, and the state directory are unreachable and unnamed.

An unconfigured mcp server starts anyway, and that is deliberate. If a client launches mail-muncher mcp before there is a config, the server does not exit — it completes the handshake, registers the same five tool names, and answers every call with the setup guidance as a tool error, so the agent has something to relay instead of "server failed to start". If you are wiring this up for an operator, that is expected behaviour and not a bug to file. The guidance also goes to stderr at startup, where clients tee the server log.

Full reference, client wiring, and every argument and return field: docs/mcp.md.

list_rules is the one that closes the loop. The agent writes a domain to its own file, then asks list_rules and sees its own subscription reflected back — the same list the next cycle will match against.

Alternatives

Read this before adopting. Several tools do the fetch-filter-deliver shape well, and some of them are a better fit than this one.

Tool	Use it instead when
getmail6	You want a mature, widely packaged fetcher. It does IMAP and Gmail OAuth2, delivers to Maildir/MDAs, and filters through external programs. If a human (or mutt, or notmuch) is the consumer, this is the stronger tool.
fdm	You want per-rule Maildir destinations with a compact, well-tested config — exactly this tool's shape, minus the external filter source. Gmail access is app-password IMAP.
lieer	You want your whole Gmail mailbox synced bidirectionally into a local Maildir for notmuch, not a filtered subset pulled out of it.
gmail-archive	It was almost exactly this — Gmail query to Maildir, incremental — and would be the obvious answer if it were still maintained. It has not been since 2018.
gmail-exporter	You want a one-off, label-based, spreadsheet-shaped export rather than incremental sync.
mbsync / offlineimap	You want full mailbox replication and will filter locally afterwards.

What none of them do, and what this tool exists for: take filter input from a file another program owns and re-read it every cycle, and emit a rendering built for a program to consume rather than for a mail client to display. If you do not need both of those, one of the tools above will serve you better and has years more mileage.

Install

No Go toolchain required for the first two options.

Homebrew

brew install craigjmidwinter/tap/mail-muncher

That taps craigjmidwinter/homebrew-tap and installs a prebuilt binary. brew upgrade mail-muncher tracks new releases.

Download a binary

Every release ships archives for macOS and Linux on both amd64 and arm64, plus a checksums.txt and a signature over it.

# Latest release, without the leading v. Set this by hand to pin a version.
VERSION=$(curl -fsSL https://api.github.com/repos/craigjmidwinter/mail-muncher/releases/latest \
  | sed -n 's/.*"tag_name": *"v\{0,1\}\([^"]*\)".*/\1/p')

OS=$(uname -s | tr '[:upper:]' '[:lower:]')     # darwin | linux
ARCH=$(uname -m | sed 's/x86_64/amd64/;s/aarch64/arm64/')

curl -fsSLO "https://github.com/craigjmidwinter/mail-muncher/releases/download/v${VERSION}/mail-muncher_${VERSION}_${OS}_${ARCH}.tar.gz"
tar xzf "mail-muncher_${VERSION}_${OS}_${ARCH}.tar.gz" mail-muncher
sudo install -m 0755 mail-muncher /usr/local/bin/mail-muncher

On macOS, a binary you downloaded yourself is quarantined by Gatekeeper. Clear it with xattr -d com.apple.quarantine /usr/local/bin/mail-muncher, or use the Homebrew install above, which does this for you.

Verify what you downloaded

This tool reads your mail. Check that the archive is the one the release workflow built. First the checksum:

curl -fsSLO "https://github.com/craigjmidwinter/mail-muncher/releases/download/v${VERSION}/checksums.txt"

# Linux
sha256sum --check --ignore-missing checksums.txt
# macOS
shasum -a 256 --check --ignore-missing checksums.txt

Then the signature over checksums.txt. Releases are signed keylessly with cosign — there is no public key to fetch and no private key anyone has to guard. The signing certificate is issued to the release workflow's own GitHub OIDC identity and recorded in the public Rekor transparency log, so what you are checking is "this was built by release.yml in this repo, from a tag":

curl -fsSLO "https://github.com/craigjmidwinter/mail-muncher/releases/download/v${VERSION}/checksums.txt.sig"
curl -fsSLO "https://github.com/craigjmidwinter/mail-muncher/releases/download/v${VERSION}/checksums.txt.pem"

cosign verify-blob \
  --certificate checksums.txt.pem \
  --signature checksums.txt.sig \
  --certificate-identity-regexp '^https://github\.com/craigjmidwinter/mail-muncher/\.github/workflows/release\.yml@refs/tags/' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  checksums.txt

Verified OK means the checksum file is authentic; the sha256sum step then ties your archive to it. cosign 3 prints a deprecation notice for --certificate and --signature — the check still runs, and these detached files are what cosign 2 understands too.

go install

The right path if you already have Go 1.25 or newer:

go install github.com/craigjmidwinter/mail-muncher/cmd/mail-muncher@latest

Note that go install builds report dev for --version, because the version is stamped at link time and the go tool does not do it. Released binaries and make build report the real tag. If you file a bug from a go install build, say which commit you installed.

Build from source

git clone https://github.com/craigjmidwinter/mail-muncher
cd mail-muncher
make build          # -> ./mail-muncher, version stamped from git describe

make snapshot builds the full set of release archives locally (requires goreleaser) if you want to check what a release would contain.

The example configs referenced below live in examples/ — imap.yml, minimal.yml and job-search.yml. They are also bundled inside every release archive, so a binary download has them too. You do not need them to get started, though: mail-muncher init writes a config from scratch.

Container image

docker pull ghcr.io/craigjmidwinter/mail-muncher:latest

linux/amd64 and linux/arm64, built from the same binaries the release archives carry. The image's default command is mcp, because serving the archive over stdio is the mode a container suits: a client starts it, talks to it, and stops it. run and daemon work too — override the command — but on a host those are a cron line and a launchd/systemd unit, which fit better.

Two mounts, and both matter:

docker run -i --rm \
  -e IMAP_PASSWORD \
  -v ~/.config/mail-muncher:/home/muncher/.config/mail-muncher:ro \
  -v ~/.local/share/mail-muncher:/home/muncher/archive \
  ghcr.io/craigjmidwinter/mail-muncher:latest mcp

Every path inside config.yml has to be a path the container can see. A dest: of ~/Mail/receipts resolves against the container's home directory, not yours, so mail lands on a layer that disappears when the container exits. Point dest: at the mounted directory — /home/muncher/archive/receipts for the mount above — or you will archive into the void and the manifest will cheerfully tell you it worked.

password_cmd runs inside the container, under /bin/sh, which means your host password manager is not there. pass show mail/fastmail cannot work. Use the secret material the container does have:

password_cmd: printenv IMAP_PASSWORD          # -e IMAP_PASSWORD
password_cmd: cat /run/secrets/imap-password  # docker secret or a mounted file

This is the one place the container path is genuinely worse than a host install: it moves the credential out of your password manager and into the container's environment. If that trade is not worth it to you, install the binary — password_cmd is designed for the host case, and this is the compromise, not the intent.

The image is also what backs the MCP Registry listing; server.json is that entry, and its name has to match the io.modelcontextprotocol.server.name label baked into the image.

As a Claude Code skill

The repo ships a skill and plugin package under skills/, which installs mail-muncher as something an agent can set up and drive for you — writing the config, running auth, and wiring the MCP server into your client. If that is how you want to adopt it, start there instead of the quickstart below.

The skill leads with provider: imap and drives mail-muncher init, so it takes the same two-minute route this README does rather than sending you to the Google Cloud Console.

Quickstart

About five minutes, no browser, no clone. This is the IMAP path; for the Gmail API instead, read Quickstart: Gmail below before you start, because it costs about ten minutes in the Google Cloud Console and the token expires weekly.

0. Check the install. Right after installing, before there is any config:

mail-muncher run

That is a genuinely useful smoke test rather than a mistake. It exits 1 and tells you exactly where it looked, what to run next, and what each provider costs:

mail-muncher is not configured.
  missing config file: /Users/you/.config/mail-muncher/config.yml
  next command:        mail-muncher init
  then:                mail-muncher validate && mail-muncher run --dry-run

init asks which provider to use. Both are supported; the costs differ.
  provider: imap   ~2 min. Gmail, Fastmail, Proton Bridge, work accounts,
    self-hosted. Needs an app password, which is a broader credential than a
    read-only OAuth token; mail-muncher only ever issues BODY.PEEK.
  provider: gmail  ~10 min in the Google Cloud Console: gmail.readonly is a
    Google restricted scope, so mail-muncher ships no OAuth client and you
    register your own. Google enforces read-only, but on a Testing-mode
    consent screen the refresh token expires every 7 days, so
    "mail-muncher auth" must be re-run weekly. Read docs/gmail-setup.md.
Docs: https://craigjmidwinter.github.io/mail-muncher/

Every command that needs a config says this, so a broken install and an unconfigured one never look alike.

1. Get an app password. From your mail provider's own settings page — Gmail, Fastmail, iCloud, Proton, your work account. It is scoped to this one use and you can revoke it without touching anything else. Put it wherever you already keep secrets:

# macOS Keychain
security add-generic-password -s mail-muncher -a "$USER" -w

# or pass, 1Password, secret-tool, gpg — anything that prints it on stdout

mail-muncher never stores this. It runs a command you name and reads the password off that command's stdout, so the secret stays in your password manager. There is deliberately no password: key in the config schema.

2. Write a config.

mail-muncher init --provider imap

Account name [personal]:
Write matched mail to [~/Mail/mail-muncher]:
Wrote /Users/you/.config/mail-muncher/config.yml

Next, for provider imap:
  1. Edit imap.host, imap.username and imap.password_cmd in:
       /Users/you/.config/mail-muncher/config.yml
     Create an app password with your mail provider and store it where
     password_cmd can read it.
  2. Run that same password_cmd in a shell and check it prints the password
     and nothing else, for example:
       security find-generic-password -s mail-muncher -w | cat -A
     Anything else on stdout - a prompt, a warning, a trailing blank line -
     becomes part of the password and the login fails.
  3. mail-muncher validate
  4. mail-muncher run --dry-run     then     mail-muncher run
Matched mail lands in ~/Mail/mail-muncher
Docs: https://craigjmidwinter.github.io/mail-muncher/

init is interactive by default and asks three questions — provider, account name, destination. Add --account NAME and --dest DIR to answer them up front, or --yes to take every default. --yes still requires --provider: the two paths cost different things, so there is no honest default to pick on your behalf. An existing config is never overwritten without --force.

The whole thing scripts, which is what an installing agent wants:

mail-muncher init --provider imap --yes

~/.config/mail-muncher/config.yml is the default path; --config overrides it everywhere, including for init.

3. Fill in three values. Open the file init just wrote and set imap.host, imap.username and imap.password_cmd. It is commented throughout, and examples/imap.yml is a fuller worked version:

accounts:
  - name: personal
    provider: imap
    imap:
      host: imap.fastmail.com
      username: you@fastmail.com
      password_cmd: security find-generic-password -s mail-muncher -w
      mailboxes: [INBOX]

Run that password_cmd in a shell first and check it prints the password and nothing else — | cat -A makes a stray prompt or trailing blank line visible. Anything extra on stdout becomes part of the password and the login fails.

4. Check the config.

mail-muncher validate

config: /Users/you/.config/mail-muncher/config.yml
1 account(s), 1 rule(s), state_dir /Users/you/.local/state/mail-muncher
OK

An IMAP account validates clean: no credential file to find, no token to have written yet, nothing on disk at all. OK with no warnings is the expected result. validate parses the config, compiles every rule's match tree, and checks the files it references. Missing files that another program owns — a from_domains_file, or on the Gmail path the OAuth credentials and token — are warnings, not errors:

warning: rules[0].match.any[0].from_domains_file: file does not exist yet: /Users/you/.local/share/jobsearch/domains.txt (it is maintained by another program; the rule matches nothing until it appears)
OK with 1 warning

5. See what a real run would do.

mail-muncher run --dry-run

A dry run connects, fetches and evaluates exactly as a real run does, and reports the path each match would be written to. It writes no files and does not save sync state, so you can run it as many times as you like. This is also where a wrong host, username or password_cmd surfaces, named exactly:

error: account "personal": imap: password_cmd "security find-generic-password -s mail-muncher -w" failed: exit status 44: security: SecKeychainSearchCopyNext: The specified item could not be found in the keychain.

6. Run it.

mail-muncher run

The config init wrote carries one starter rule matching everything newer than 72h, so this first run is guaranteed to store something — a run that stores nothing is indistinguishable from a broken install. Then run it again: everything already on disk reports as skipped, and the incremental cursor means the second run barely talks to the server at all.

Once that works, narrow the starter rule into what you actually want (docs/filters.md), then put it on a schedule (see Scheduling). docs/configuration.md has every key.

Quickstart: Gmail

Take this path if you specifically want the Gmail API and a read-only guarantee enforced by Google rather than by this program. Know the two costs before you begin, because both are structural and neither goes away:

• About ten minutes in the Google Cloud Console, up front. gmail.readonly is a Google restricted scope, so mail-muncher ships no OAuth client and never will — you register your own project and Desktop-app client and download its JSON.
• The token expires every 7 days. Google applies that to every consent screen still in Testing mode, which yours will be. mail-muncher auth has to be re-run weekly. There is no setting that removes it; docs/gmail-setup.md explains why and what the alternatives cost.

If neither is worth it to you, the IMAP path above works on a Gmail account.

mail-muncher init --provider gmail        # prints the cost warning, then writes the config
# → follow docs/gmail-setup.md: project, Gmail API, consent screen,
#   Desktop app OAuth client, save its JSON as
#   ~/.config/mail-muncher/credentials.json
mail-muncher auth --account personal      # browser consent; writes token.json 0600
mail-muncher validate
mail-muncher run --dry-run
mail-muncher run

auth prints a consent URL (and tries to open a browser), listens on a loopback port for the redirect, and writes the token to the account's token_file with mode 0600. It is a Gmail-only command — on an IMAP account it refuses, because there is nothing to authorize. Steps 4 through 6 of the IMAP quickstart above then apply unchanged; validate will report two warnings until the credentials and token files exist.

Externally-managed filter files

This is the feature the tool is built around, so it is worth being precise about the semantics.

from_domains_file names a file that mail-muncher does not own, does not create, and never writes:

match:
  from_domains_file: ~/.local/share/jobsearch/domains.txt

# ~/.local/share/jobsearch/domains.txt
# written by the job-search tracker

acme.com
globex.io          # inline comments are fine
@initech.com       # a leading @ is stripped
MAIL.Umbrella.COM  # case is irrelevant

• Read once per cycle, on first use. Not once per process, and not once per message. run re-reads it; every daemon tick re-reads it. A file referenced by several rules is read once and shared.
• Missing or unreadable is never fatal. The predicate simply matches nothing and one warning is logged for that file for that cycle. The owning program may not have created it yet, and mail-muncher must not fail because of that.
• Liberal parsing. One entry per line; # starts a comment; blank lines are skipped; surrounding whitespace is trimmed; a leading @ and a trailing . are stripped; everything is lowercased; duplicates collapse. An entry with no dot in it is kept and logged as suspicious rather than dropped, because the file belongs to someone else and guessing wrong should not silently discard an entry.
• Equality or subdomain. acme.com matches acme.com and careers.acme.com, but not notacme.com.

The same matching rules apply to the inline from_domains: predicate; the only difference is who owns the list.

When a domain list cannot express it

Some senders cannot be enumerated in advance. One company's mail might arrive from wagepoint.teamtailor.com, mail.wagepoint.com and notifications@wagepoint-hr.example — a domain list can only name hosts you already know about. from_regex_file is the same idea for patterns:

match:
  from_regex_file: ~/.local/share/jobsearch/companies.txt

# one RE2 pattern per line, unanchored
wagepoint
(?i)^careers@acme\.io$
teamtailor\.com$

Lifecycle is identical — read once per cycle, missing is never fatal, on_degraded_filter governs the cursor. Two deliberate differences from the domain format:

• Nothing is lowercased, because a regex is case-sensitive by construction. Write (?i) when you want otherwise.
• # only starts a comment at the start of a line. Truncating a pattern at a mid-line # would silently change what it matches.

The failure modes are opposites, and that is why the guards differ. A typo in a domain list matches nothing — the cost is silence. A typo in a pattern list can match everything: .* or a stray blank claims the entire mailbox. So an empty pattern, or any pattern that matches the empty string, is refused outright; a pattern that fails to compile is refused by itself while the rest of the file stays in force; and the count of patterns loaded is logged every cycle, so a file that fell from twelve patterns to one catch-all is a number in your run output rather than a discovery by way of a full disk.

Configuration

Full reference: docs/configuration.md. Runnable files: examples/imap.yml, examples/minimal.yml, examples/job-search.yml.

The account block is the only part that differs by provider. IMAP:

accounts:
  - name: personal
    provider: imap
    imap:
      host: imap.fastmail.com
      port: 993                  # default
      tls: true                  # default
      username: you@fastmail.com
      password_cmd: pass show mail/fastmail   # stdout is the password
      mailboxes: [INBOX, Archive]             # default [INBOX]
      initial_lookback: 720h                  # default

Gmail:

state_dir: ~/.local/state/mail-muncher

on_message_failure: quarantine   # or: abort
on_degraded_filter: hold         # or: fail, proceed

accounts:
  - name: personal
    provider: gmail
    gmail:
      credentials_file: ~/.config/mail-muncher/credentials.json
      token_file: ~/.config/mail-muncher/token.json
      query: "-in:chats"
      initial_lookback: 2160h

rules:
  - name: job-search
    account: personal
    match:
      any:
        - from_domains_file: ~/.local/share/jobsearch/domains.txt
        - subject_regex: "(?i)your application"
    dest: ~/Mail/job-search
    formats: [eml, markdown]

Key	Type	Default	Description
state_dir	path	~/.local/state/mail-muncher	Sync cursors (one JSON file per account), the cycle lock, the instance lock, and the quarantine directory.
on_message_failure	quarantine, abort	quarantine	What to do with a message that will not parse or that a sink failed on. See below.
on_degraded_filter	hold, fail, proceed	hold	What to do when a rule's from_domains_file or from_regex_file cannot be read. See below.
quarantine_dir	path	<state_dir>/quarantine	Where quarantined messages are parked.
accounts	list	—	Mailboxes to pull from. At least one is required.
accounts[].name	string	—	Required, unique. Names the state file and is what rules[].account refers to.
accounts[].provider	imap, gmail	—	Required; there is no default. Which backend fetches. See Two ways to connect a mailbox.
accounts[].imap	mapping	—	Required — and only permitted — when the provider is imap.
accounts[].imap.host	string	—	Required. imap.fastmail.com, imap.gmail.com, 127.0.0.1 for the Proton Bridge.
accounts[].imap.port	integer	993	993 is implicit TLS (IMAPS) and pairs with the tls: true default.
accounts[].imap.username	string	—	Required. Usually the full address; some providers want the bare local part.
accounts[].imap.password_cmd	shell command	—	Required. Run under /bin/sh -c; its stdout is the password. There is deliberately no password key — the secret stays in your password manager.
accounts[].imap.mailboxes	list of strings	[INBOX]	Folders to fetch, each with its own cursor. A name doubles as the label predicate value. A folder the server does not have is an error, not an empty folder.
accounts[].imap.tls	boolean	true	Implicit TLS on connect. false sends the password and every body in the clear; validate warns. Legitimate only on loopback or behind an stunnel.
accounts[].imap.initial_lookback	Go duration	720h	How far back a first-ever sync of each mailbox reaches, and again after any UIDVALIDITY change. Must be positive.
accounts[].gmail	mapping	—	Required — and only permitted — when the provider is gmail.
accounts[].gmail.credentials_file	path	—	Required. The OAuth client JSON downloaded from Google Cloud.
accounts[].gmail.token_file	path	—	Required. Where auth caches the OAuth token, mode 0600.
accounts[].gmail.query	string	none	Optional Gmail search expression. A cost optimization for the first-ever scan only — see below.
accounts[].gmail.initial_lookback	Go duration	720h	How far back the first-ever scan reaches. Must be positive. See Backfill.
accounts[].gmail.include_spam_trash	boolean	false	Fetch messages in Spam and Trash. Honoured identically by both Gmail sync paths. validate warns when true. See Spam and Trash.
rules	list	—	Evaluated in order against every message; first match wins.
rules[].name	string	—	Required, unique. Appears in logs and in markdown frontmatter.
rules[].account	string	all accounts	Restricts the rule to one account.
rules[].match	match node	—	Required. See Filters.
rules[].dest	path	—	Required. Destination directory; created on demand.
rules[].formats	list of eml, markdown	[eml]	Renderings to write.

Notes that bite people:

• Unknown keys are a hard error. A typo fails the load rather than being ignored, so validate catches initial_lookbak before a run does.
• ~ and $VAR are expanded in every path-valued field, including from_domains_file and from_regex_file values inside a match tree. ~user forms are not supported. An undefined variable expands to the empty string, as in a shell.
• gmail.query does not filter what gets kept, and applies to less than you think. It is sent to Gmail on the first-ever scan of an account and nowhere else — not on incremental cycles, and not on a recovery scan after the history cursor expires. It is never re-applied locally. Your rules are the only authority on what is stored. Keep the query broad, or omit it.
• Spam and Trash are not fetched by default (Gmail). Both Gmail sync paths agree on this: full scans pass includeSpamTrash=false, and the incremental path drops messages labelled SPAM or TRASH before they reach the pipeline. Set gmail.include_spam_trash: true to fetch them anyway — see Spam and Trash. On IMAP there is no equivalent key: you fetch exactly the folders you list in mailboxes:, so simply not listing the junk folder is the whole mechanism.
• The gmail: and imap: blocks are mutually exclusive. Setting the one that does not match provider: is a hard error rather than a silently ignored block, so an imap: block under a Gmail account cannot leave you believing you are fetching over IMAP when you are not.

Policies for the two things that can go wrong

Both keys sit at the top level, beside state_dir. The defaults are the safe choices; you only change them if you have decided which failure you prefer.

on_message_failure — a message that will not parse, or where every rendering its rule asked for failed to write.

Value	Behavior
quarantine (default)	Write the raw bytes to <quarantine_dir>/<account>/<id>.eml with a .json sidecar naming the failure, then let the cursor advance past the message. Nothing is lost, and one poison message cannot wedge the pipeline. Counted as quarantined in the summary and manifest; the run still exits 0.
abort	Return the failure, so the cursor does not advance and the message is re-fetched next cycle. The trade-off is explicit: a permanently unparseable message wedges the account until a human deals with it.

A quarantine write that itself fails falls back to abort semantics for that message — refusing to advance is recoverable, losing the message is not.

on_degraded_filter — a rule's from_domains_file or from_regex_file is missing, unreadable, or truncated partway through. Such a file matches nothing, so without a policy every message that cycle would be evaluated against an empty list, found not to match, and consumed.

Value	Behavior
hold (default)	Run the cycle and store everything that did match, log the degradation at error level, but do not save the advanced cursor — so the same mail is re-evaluated once the file returns. The manifest reports degraded and state_held. Exit 0.
fail	End the cycle before anything is fetched. Nothing stored, nothing advanced, non-zero exit.
proceed	Treat an unreadable list as an empty one and advance anyway. The old behavior, and the only option that accepts silent loss of wanted mail — validate warns about it.

Files already stored under hold stay stored: the sinks are idempotent, so the re-run skips them.

Filters

Full reference and cookbook: docs/filters.md.

A match: value is a mapping with exactly one key — a combinator or a predicate. Two keys in one mapping is a compile error that tells you to combine them with all: or any:. Regexes and durations are compiled when the config loads, so a bad pattern is a validate failure, not a surprise at 3am.

Combinators

Key	Value	Matches when
all	list of nodes	every child matches (at least one child required)
any	list of nodes	at least one child matches (at least one child required)
not	a single node	the child does not match

match:
  all:
    - any:
        - from_domains: [acme.com]
        - from_domains_file: ~/.local/share/agent/domains.txt
    - not:
        subject_regex: "(?i)^\\[newsletter\\]"

Predicates

Key	Value	Matches when
from_domains	list of domains	any From address's domain equals or is a subdomain of a listed domain
from_domains_file	path	same, with the list read from an externally-owned file each cycle
from_regex	RE2 pattern	the pattern matches any From addr-spec (no display name)
from_regex_file	path	same, with the patterns read from an externally-owned file each cycle
to_regex	RE2 pattern	the pattern matches any To or Cc addr-spec
subject_regex	RE2 pattern	the pattern matches the decoded Subject
header	{name: X-Foo, regex: ...}	the pattern matches any value of that header
has_attachment	true / false	the message does (or does not) carry a real attachment
label	label name	the message carries that provider label, compared exactly. On Gmail that is a Gmail label; on IMAP it is the name of the mailbox the message came from
older_than	Go duration	the message Date is further in the past than the duration
newer_than	Go duration	the message Date is more recent than the duration

One worked example each:

# Mail from a company or any of its subdomains.
- from_domains: [acme.com, globex.io]

# The same list, owned and updated by another program.
- from_domains_file: ~/.local/share/jobsearch/domains.txt

# A specific sender, however they capitalize it.
- from_regex: "(?i)^no-?reply@acme\\.com$"

# Patterns owned and updated by another program, for senders whose host cannot
# be enumerated in advance (wagepoint.teamtailor.com, mail.wagepoint.com).
- from_regex_file: ~/.local/share/jobsearch/companies.txt

# Anything addressed to a plus-alias you hand out to vendors.
- to_regex: "(?i)^me\\+vendors@example\\.com$"

# Application acknowledgements, case-insensitively.
- subject_regex: "(?i)(your application|application received)"

# Everything a mailing list tags for you.
- header: {name: List-Id, regex: "golang-nuts"}

# Only messages that actually carry a file.
- has_attachment: true

# On Gmail: labels exactly as shown in the UI. Nested labels use "Parent/Child";
# system labels are upper case (INBOX, SENT, UNREAD, STARRED).
# On IMAP: the mailbox the message came from, verbatim as the server names it,
# including its hierarchy separator ("Lists/golang", "Lists.golang").
- label: INBOX

# Message Date older than 90 days / newer than a day.
- older_than: 2160h
- newer_than: 24h

Details worth knowing:

• from_regex and to_regex test the bare address (jane@acme.com), never the display name. Use header: {name: From, regex: ...} to test the raw header including the display name.
• has_attachment counts parts marked Content-Disposition: attachment. Inline images referenced by cid: are not attachments.
• label is case-sensitive and exact — label: inbox does not match INBOX. On an IMAP account the values are the mailbox names you listed under imap.mailboxes, so a message can carry only the one it was fetched from.
• older_than / newer_than compare against the message Date header, falling back to the provider's internal date when the header is missing or unparseable. A message with no usable date matches neither.
• Patterns are Go RE2: no backreferences and no lookaround. Prefix with (?i) for case-insensitivity. In YAML, prefer double quotes and escape backslashes ("\\."), or use single quotes where no escaping is needed.
• Use true / false for has_attachment. YAML 1.2 treats yes and no as strings, and mail-muncher rejects them.

Spam and Trash (Gmail)

Spam and Trash are not fetched by default. Nothing in those folders reaches your rules, and nothing lands on disk. Spam is the likeliest source of hostile, attacker-authored text in a pipeline that ends in a model's context window, so the default is to leave it where Gmail put it.

This whole section is about the Gmail provider. IMAP has no equivalent key because it needs none: an IMAP account fetches exactly the folders named in imap.mailboxes, so junk arrives only if you ask for it by name.

If you want it anyway — a legitimate message misfiled as spam, or an archive that is genuinely complete — set the key per account:

accounts:
  - name: personal
    gmail:
      include_spam_trash: true   # validate warns; that is deliberate

The two settings do different jobs, and you may want both:

	Decides
gmail.include_spam_trash	whether those messages are fetched at all
A rule on the SPAM / TRASH labels	what happens to them once fetched

gmail.query cannot do either job. It is sent only on the first-ever scan, so -in:spam there does nothing for any later cycle. With include_spam_trash: true set, discriminate with a rule — the filter engine is the only thing that sees every fetched message:

rules:
  - name: job-search
    match:
      all:
        - from_domains_file: ~/.local/share/jobsearch/domains.txt
        - not:
            any:
              - label: SPAM
              - label: TRASH
    dest: ~/Mail/job-search

Gmail's system labels are exact and upper case. If you want Spam and Trash out of every rule, put the not: in each one — there is no global exclusion, by design: rules are the single authority on what is stored.

On-disk layout

The files mail-muncher writes are its public API. This section is the tour; docs/output-format.md is the contract — every frontmatter key, why the frontmatter needs a real YAML parser, and the rules for enumerating a delivery tree safely. Read it before you write a consumer, and see examples/read_delivered.py for a short correct one.

Every sink files a message under the rule's dest by the message date, in UTC:

~/Mail/job-search/
└── 2026/
    └── 07/
        ├── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.eml
        ├── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.md
        └── 1785230100-a00d5c5e383a1c08-re-your-application-for-senior-engineer.attachments/
            ├── offer.pdf
            └── R-sum-2026.docx

The basename is shared by every format, so a message's renderings sort together:

<unix-seconds>-<sha256(account + ":" + message-id)[:16]>-<subject-slug>

The digest fragment is 16 hex characters — 64 bits. Two messages colliding on it is not reachable at any volume a mailbox produces, and readers of the archive parse it back out as a message id, so treat the width as part of the layout.

• The timestamp sorts a directory chronologically.
• The digest is the idempotency key. It depends only on the account name and the provider message id, so the path is a pure function of message identity.
• The slug is the subject lowercased, with every character outside [a-z0-9] collapsed to a single -, trimmed, and truncated to 40 characters.

Two caveats about the slug, both deliberate:

• It is ASCII-only. A subject written entirely in a non-Latin script, or entirely in emoji, slugs to no-subject. Non-ASCII filenames would be subject to filesystem Unicode normalization (HFS+ stores NFD), which can make the name written differ from the name the next cycle checks for — and that existence check is the entire idempotency story. The digest still keeps such messages apart.
• It is cosmetic. Only the digest carries identity. Two messages with the same subject never collide.

How files are written

A message file is written to a temp file in its destination directory, fsynced, and then hard-linked into place with link(2). Three consequences worth relying on:

• A partial file is never published. The temp file is complete before the name exists.
• An existing file is never overwritten. link(2) fails with EEXIST rather than clobbering, unlike rename(2). That failure is the idempotency check — the kernel decides whether the name is free at the instant it is claimed, so there is no window in which another writer can slip a file in and have it silently replaced. "Already there" is reported as skipped.
• Symlinks are refused, not followed. A symlink at a message's final path, or standing in for the <YYYY> or <MM> directory below dest, is an error the message is counted and logged for. Nothing in the layout is legitimately a link, so one means something else is placing them there. The rule's own dest: is exempt — pointing that at another volume is ordinary.

On a filesystem without hard links (FAT, some network mounts) the fallback is an O_CREAT|O_EXCL write in place: still atomically no-clobber and still symlink-proof, at the cost of the no-partial-file guarantee.

Attachments are the one exception: they are written with a temp file and rename(2), because their names are not the idempotency marker — the .md above them decides that.

Directories are created 0700 and files 0600. Archived mail is private correspondence and decoded attachments, so it gets the same treatment as the sync cursors and the OAuth token: nothing here is readable by other local users. A tool you run as yourself is unaffected.

The .eml file

model.Message.Raw, byte for byte, exactly as the provider delivered it. Nothing is re-encoded, re-wrapped, or normalized, so it round-trips through any mail tool and still verifies against DKIM signatures. This is the fidelity copy.

The .md file

YAML frontmatter, then the body, then links to any attachments.

• Body selection: the text/plain part if there is one; otherwise the text/html part converted to markdown; otherwise the literal *(no body)*. Line endings are normalized to LF, trailing whitespace is stripped per line, and leading and trailing blank lines are trimmed.
• Frontmatter always carries subject, from, from_address, from_addresses, to, to_addresses, date, message_id, thread_id, thread_id_source, account, rule. cc, cc_addresses, in_reply_to, labels and attachments are omitted when empty. Parse addresses from the *_address / *_addresses fields, never from from/to/cc — those are display strings, and a sender-chosen display name containing <, > or , makes them ambiguous. The machine-readable fields carry bare addr-specs and cannot be spoofed that way. It is produced with a YAML encoder, not string formatting, so a subject full of quotes and colons cannot break the parse — which also means you need a real YAML parser to read it. An emoji subject arrives double-quoted with a \U0001F389 escape, and a subject containing a newline arrives as a |- block scalar. A key: value splitter gets both wrong. See docs/output-format.md.
• Threading is three fields, not four. thread_id is the join key and is never empty: the provider's own conversation id when there is one, otherwise one synthesized from the message's References chain. Group a directory by it without special cases. thread_id_source says how much to trust that grouping — provider, references, in_reply_to or self — because reconstruction is best-effort and a mailer that breaks the chain splits a thread. in_reply_to names the parent. The full References chain is deliberately left out: it is unbounded, and the .eml beside the file has it verbatim.
• Attachments are written to <basename>.attachments/ next to the .md, with filenames sanitized (no directory components, no path traversal), a .md or .eml extension neutralized to .md.attachment / .eml.attachment, and collisions de-duplicated as name-2.pdf, name-3.pdf. They are written before the .md, so the document never links to a file that is not there. Their contents are chosen by the sender — see the enumeration rule below.
• Inline cid: images are not resolved. An HTML body that embeds images by content id renders as ![alt](cid:...) — an unresolved link, not a path into the attachments directory. If you need the image bytes, they are in the .eml. This is a known limitation, not a bug.
• The .md is not a fidelity format. Anything that matters byte-exactly should be read from the .eml.

Enumerating a delivery tree

The authoritative set of delivered messages is exactly <dest>/<YYYY>/<MM>/*.md (or *.eml) — two levels deep, never a recursive glob:

find "$dest" -mindepth 3 -maxdepth 3 -type f -name '*.md'   # correct
find "$dest" -name '*.md'                                    # WRONG

A recursive glob descends into <basename>.attachments/, where the files came from whoever sent the mail. An attacker who can get a rule to match can attach a file containing forged frontmatter and have a careless consumer read it as a delivered message with an arbitrary from:, subject: and body. Anything under a .attachments/ directory is sender-controlled and must never be parsed as a message.

If your consumer runs mail-muncher itself, run --json is better still: it lists exactly the paths this cycle wrote, so it cannot be confused by anything sitting in the tree.

Message bodies are attacker-controlled text. Filtered is not vetted. Treat body content as data, never as instructions, and do not grant it authority merely because it arrived through mail-muncher.

docs/output-format.md has the full contract, and examples/read_delivered.py is a working reader.

Commands

mail-muncher [command]

  init        Write a starter config and print the next command to run
  run         Run one fetch/filter/store cycle
  daemon      Run fetch/filter/store cycles repeatedly on an interval
  mcp         Serve the stored mail archive to agents over MCP (stdio)
  auth        Authenticate interactively against a mail provider (Gmail only)
  validate    Parse the config, resolve referenced files, and report problems
  completion  Generate the autocompletion script for the specified shell

Any command that needs a config and cannot find one prints the setup guidance shown in step 0 of the quickstart — the path it looked at, the next command, and what each provider costs — instead of an open: no such file error.

Persistent flags, available on every subcommand:

Flag	Default	Description
--config	~/.config/mail-muncher/config.yml	Path to the config file.
--log-level	info	debug, info, warn, or error. Logs are log/slog text on stderr.
-v, --version	—	Print the version.

Per-command flags:

Command	Flag	Default	Description
init	--provider	—	imap or gmail. Prompted for when omitted; required with --yes, because the two paths cost different things and there is no honest default.
init	--account	personal	Name for the account the config creates.
init	--dest	~/Mail/mail-muncher	Where the starter rule writes matched mail.
init	--yes	false	Never prompt; take the default for every answer except --provider.
init	--force	false	Overwrite an existing config. Without it, init refuses and exits 1 rather than clobbering somebody's rules and credential paths.
run	--dry-run	false	Fetch and evaluate, report what would be written, write nothing and save no state.
run	--json	false	Write a machine-readable manifest to stdout, one JSON object per account.
daemon	--interval	5m	Time between cycles, minimum 30s. Each sleep is jittered by up to ±10%.
daemon	--dry-run	false	As run --dry-run, on every tick.
daemon	--json	false	Write a manifest to stdout after every cycle: newline-delimited JSON, one object per account per tick.
auth	--account	—	Which account to authenticate. Required when the config has more than one. Gmail accounts only — on an IMAP account auth refuses, because there is nothing to authorize.

mcp takes no flags of its own; it is configured entirely by --config. Note that init writes to --config rather than reading from it, and creates the directory (0700) and the file (0600) as needed.

--log-level debug logs the rule decision for every message — the message id, its subject, and the winning rule name or no match — which is the fastest way to work out why a rule is not firing. Debug also logs each already stored skip with its path. All of it goes to stderr, so it never disturbs --json on stdout.

An interval below 30s is rejected before anything starts:

error: --interval 10s is below the 30s minimum

Exit codes

Code	Meaning
0	Success. A cycle that hit per-message errors (a message that would not parse, one sink write that failed) still exits 0 — those are counted in the summary, not escalated.
1	Config or validation error: the file would not parse, a rule would not compile, a required key is missing, or there is no config file at all. Nothing was fetched. Also what init returns when a config already exists and --force was not given.
2	Provider or authentication failure: a Gmail token that was never written or that Google rejected, an IMAP password_cmd that failed or a login the server refused, the server unreachable after retries.
3	Another instance holds the cycle lock. Exit code 3 is how an overlapping cron invocation reports "the previous one is still going".

A quarantined message does not change the exit status: it is a message-level failure that was handled, counted as quarantined, and reported in the manifest. Alert on the counter, not on the exit code.

daemon never exits 2. A failing cycle is logged with a consecutive-failure count and retried on the next tick — a daemon that quit on the first expired token would need a human at exactly the wrong moment. Its statuses are 0 (stopped by SIGINT or SIGTERM, after letting the in-flight cycle finish and saving state), 1 (config or validation error, including an --interval below the minimum), and 3 (another instance already held the instance or cycle lock at startup). A lock held by a later tick — a cron run overlapping a daemon tick — is logged and skipped, not fatal.

The run summary

Every cycle logs and prints one summary line per account:

personal: fetched=128 matched=6 stored=6 skipped=0 parse_errors=0 sink_errors=0 quarantined=0 duration=4.1s

Field	Counts	Meaning
fetched	messages	Messages the provider delivered this cycle.
matched	messages	Messages some rule claimed.
stored	renderings	Renderings actually written.
skipped	renderings	Renderings not written because the destination already existed.
parse_errors	messages	Messages that would not parse; logged and skipped.
sink_errors	renderings	Write failures; logged and counted, cycle continues.
quarantined	messages	Messages parked under the quarantine directory because they could not be delivered.
vanished	messages	Messages the provider listed and then found no longer existed — deleted between the listing and the download. Skipped, and the cursor advanced past them. Only printed when non-zero.

The field list runs contiguously from fetched= to duration=, so anything unusual about the cycle is marked on the account label instead:

personal (dry-run): fetched=42 ...
personal (degraded, state held): fetched=42 ...
personal (stopped): fetched=12 ...

vanished=N is the one counter appended conditionally. It appears after quarantined= only in the runs where a message disappeared mid-cycle, and is omitted everywhere else, so the published field list is unchanged for every other run. Under --json it is summary.vanished, likewise omitted when zero — parse it as .summary.vanished // 0 if you want to alert on it.

A steady-state cron run looks like fetched=0. A re-run over the same window looks like matched=N stored=0 skipped=N — that is idempotency working.

The JSON manifest

run --json and daemon --json replace that line with a machine-readable object per account — stored[], skipped[], quarantined[], the counters, and whether the cursor advanced. stderr keeps every log line, so run --json 2>/dev/null is pure JSON and daemon --json is an NDJSON stream.

mail-muncher run --json 2>/dev/null | jq -r '.stored[] | "\(.rule)\t\(.path)"'

Full field-by-field contract: docs/manifest.md.

State and locking

~/.local/state/mail-muncher/
├── personal.json                # one per account, mode 0600
├── mail-muncher.lock            # cycle lock, shared by run, daemon and mcp sync
├── instance/
│   └── mail-muncher.lock        # daemon lifetime lock — one daemon per state dir
└── quarantine/
    └── personal/
        ├── 18f2a1b2c3.eml       # raw bytes of an undeliverable message
        └── 18f2a1b2c3.json      # why it is here

Each account's state file is JSON. The cursor inside it is the provider's, so its shape differs. Gmail:

{
  "history_id": 918273,
  "last_sync_time": "2026-07-28T09:15:00Z",
  "seen_ids": ["18f2a...", "18f2b..."]
}

• history_id is the Gmail incremental cursor. When it is set, a cycle asks Gmail only what changed since. Gmail keeps roughly a week of history; when the cursor ages out the API answers 404, mail-muncher logs a warning, clears the cursor, and falls back to a full scan in the same cycle.
• last_sync_time bounds the after: term of a full scan. A recovery scan reaches 24 hours further back than the stored watermark, so mail that arrived while the previous cycle was listing is not stepped over. The overlap is harmless: everything already on disk is skipped at the write.
• seen_ids is a FIFO set of the last 2000 delivered message ids — belt and braces alongside the sinks' idempotent filenames.

IMAP tracks each mailbox independently, as a UIDVALIDITY/UID pair under extra:

{
  "last_sync_time": "2026-07-28T09:15:00Z",
  "extra": {
    "imap.INBOX.uidvalidity": "1650000000",
    "imap.INBOX.last_uid": "48213"
  },
  "seen_ids": ["personal:INBOX:1650000000:48213"]
}

A cycle whose stored UIDVALIDITY still matches the server's asks for UID FETCH 48214:* — only what arrived since. A cycle that finds it changed throws the stored UID away and resyncs from initial_lookback, because UIDVALIDITY changing is the protocol announcing that every UID mail-muncher remembers now names a different message. That resync re-archives the window it covers under fresh filenames, since a message's identity on this path is <account>:<mailbox>:<uidvalidity>:<uid>. The trade is deliberate: duplicated mail can be deleted, silently skipped mail cannot be recovered. Full detail: docs/configuration.md.

The state directory is created 0700 and state files 0600: knowing which accounts exist and which message ids were seen is not public information.

Deleting an account's state file forces a full re-scan bounded by initial_lookback. That is safe — the sinks skip everything already on disk — and it is the supported way to recover from a corrupted cursor.

The two locks

• The cycle lock, <state_dir>/mail-muncher.lock, is flock-based and held for the duration of each cycle by run, by every daemon tick, and by the MCP sync tool. It is what stops a cron invocation racing a running daemon on the same cursors. Finding it held is "not now", not "broken": run exits 3, and a daemon tick logs and skips.

• The instance lock, <state_dir>/instance/mail-muncher.lock, is held by a daemon for its whole process lifetime. The cycle lock cannot do this job — it is released between ticks, so a second daemon starting while the first sleeps would sail past it and then poll forever alongside it, doubling the API traffic against the same cursors. A second daemon exits 3 immediately:

  another mail-muncher daemon is already running against this state directory; not starting
  

Both are released by the OS if the process crashes.

Quarantine

Under the default on_message_failure: quarantine, a message that could not be delivered is written to <quarantine_dir>/<account>/<id>.eml — the raw bytes, verbatim — with a .json sidecar beside it naming the rule, the stage that failed (parse or sink), the error, and the time. The sidecar is self-contained on purpose: sweeping the directory must not require the run's logs.

Nothing re-delivers automatically. Fix the cause and feed the .eml back in by hand. The directory is 0700 and the files 0600, like the rest of the state directory — a quarantined message is a whole email sitting outside its destination tree.

Scheduling

cron — one cycle every ten minutes. run exits 3 if the previous invocation is still going, so overlaps are harmless:

*/10 * * * * /usr/local/bin/mail-muncher run --config /home/you/.config/mail-muncher/config.yml >> /home/you/.local/state/mail-muncher/cron.log 2>&1

Use an absolute path to the binary and to the config: cron's environment is not your shell's, and ~ expansion in the config depends on $HOME being set.

A fuller sample with the environment cron does not give you is in contrib/crontab.sample.

launchd (macOS) — run the daemon under launchd so it survives logout and restarts on failure. A ready-to-edit plist lives in contrib/launchd/; copy it into ~/Library/LaunchAgents/, edit the paths to the binary and config, then:

launchctl load ~/Library/LaunchAgents/<the-plist-you-copied>

systemd — no unit ships yet; mail-muncher daemon --interval 5m is a straightforward Type=simple service, or pair mail-muncher run with a timer.

Backfill: the first run

A wide first run is an intended mode, not an abuse. The default initial_lookback is 720h (30 days), which quietly means the thing you most want on day one — the whole history of what you are collecting, on disk, once — is the thing that does not happen by default.

Set it wide for the first run. The key exists on both providers, under whichever block your account uses:

accounts:
  - name: personal
    gmail:
      credentials_file: ~/.config/mail-muncher/credentials.json
      token_file: ~/.config/mail-muncher/token.json
      initial_lookback: 13140h   # ~18 months — first run only

accounts:
  - name: personal
    provider: imap
    imap:
      host: imap.fastmail.com
      username: you@fastmail.com
      password_cmd: pass show mail/fastmail
      initial_lookback: 13140h   # ~18 months — first run only, per mailbox

Then run it once, and drop the value back afterwards:

      initial_lookback: 720h

Dropping it back is safe because initial_lookback only ever bounds the first-ever scan — of an account on Gmail, of each mailbox on IMAP. Every later cycle resumes from the stored cursor and ignores the key entirely. It is re-armed only by deleting the account's state file, or, on IMAP, by the server changing UIDVALIDITY.

What to expect:

• It is one large full scan, rate-limited and retried, and it may take a while. On Gmail, downloads run four at a time and pages are 500 messages; neither is configurable. On IMAP it becomes a SINCE UID search per mailbox.

• Look before you commit. mail-muncher run --dry-run fetches and evaluates exactly as a real run does and reports every path it would write, without touching the destination tree. Add --json to count them:

  mail-muncher run --dry-run --json 2>/dev/null | jq '.summary'
  

• The run after it should report everything as skipped. That is the check that the backfill actually landed.

Go durations have no day or year unit — multiply hours. 18 months is 13140h, a year is 8760h, 90 days is 2160h.

The 2000-entry seen-set cap does not limit a backfill. See docs/architecture.md for why: the deterministic filename plus skip-on-exists is the real idempotency key, and the seen set is only belt and braces.

Status and scope

Pre-1.0. The current release is v0.1.0; a build without make reports its version as dev. The config schema is stable enough to write against, but treat it as subject to change until 1.0.

Area	Status
IMAP provider (password_cmd, per-mailbox UID cursors, EXAMINE + BODY.PEEK[])	Built
Gmail provider (OAuth, full scan, incremental history sync, RAW download)	Built
init — writes a validated config for either provider, interactive or scripted	Built
Setup guidance on every unconfigured or half-configured command	Built
mcp serving protocol-correctly when unconfigured	Built
Config loading and validation	Built
Filter engine (all combinators and predicates listed above)	Built
.eml and markdown sinks	Built
run, daemon, cycle lock, instance lock	Built
--json run manifests	Built
mcp — stdio MCP server, five tools	Built
Quarantine and the two failure policies	Built

Deliberately out of scope

• Writing to your mailbox. Read-only is a design constraint, not a phase. No labeling, no deletion, no sending, no drafts, and on IMAP not even marking a message read.
• Being a mail client. No UI, and no index. search_messages is a case-insensitive substring scan over the stored files, not a search engine: no stemming, no ranking, no query language. Threads are grouped by an id carried on each message, not reassembled into a conversation model.
• A network API. Nothing listens on a socket. mcp speaks a stdio protocol to a client that launched it as a subprocess — no port, no HTTP endpoint, no network surface. The only thing that ever binds is the loopback port auth opens for a few seconds during the OAuth redirect on the Gmail path.
• Bundling an OAuth client. gmail.readonly is a Google restricted scope, so the Gmail path will always mean registering your own — which is exactly why IMAP exists as the low-friction way in.

Known limitations

• Inline cid: images are left as unresolved links in markdown output.
• Subject slugs are ASCII-only; non-Latin subjects file as no-subject.
• No predicate sees the message body; matching is on headers, labels and dates.

Gmail:

• The Gmail token expires every 7 days on a Testing-mode consent screen, so mail-muncher auth is a weekly chore. This is Google's policy, not a setting.
• gmail.query applies only to the first-ever scan of an account — not to incremental cycles, and not to a recovery scan after the cursor expires.
• Spam and Trash are not fetched at all unless gmail.include_spam_trash: true. Once fetched, a rule on the SPAM and TRASH labels decides what happens to them.
• Gmail's download concurrency (4) and page size (500) are not configurable.

IMAP:

• An app password is a full mail credential. Read-only is enforced by mail-muncher's code, not by your provider — see the comparison.
• A server changing UIDVALIDITY re-archives the initial_lookback window under fresh filenames. Deliberate: duplicated mail is recoverable, skipped mail is not.
• IMAP has no server-side conversation id, so thread_id is always synthesized from the References/In-Reply-To chain and thread_id_source is never provider.
• A mailbox the server does not have is an error, not an empty folder — so a typo in mailboxes: fails the first run rather than looking like quiet mail.
• Only the folders listed in mailboxes: are fetched; there is no "everything" option and no server-side search.

Documentation

Browsable at https://craigjmidwinter.github.io/mail-muncher/, or in this repo:

• docs/configuration.md — every config key for both providers, its validation rule, and its failure mode. The accounts[].imap section is the IMAP reference; there is no separate setup page because there is no setup beyond an app password.
• docs/gmail-setup.md — the Gmail path only: Google Cloud walkthrough, the seven-day expiry, and every OAuth error message with its fix. Nothing in it applies to an IMAP account.
• docs/filters.md — the complete match-tree language plus a cookbook of real rules.
• docs/output-format.md — the on-disk contract: layout, filenames, every frontmatter key, and the security rules for enumerating a delivery tree. Read it before writing a consumer.
• docs/manifest.md — the --json manifest contract, field by field.
• docs/mcp.md — the MCP server: client wiring and every tool's arguments and return shape.
• docs/architecture.md — the pipeline, its seams, and where your change belongs.
• CONTRIBUTING.md — build, test, and the conventions the codebase holds itself to.

Runnable configs: examples/imap.yml (the IMAP path, heavily commented), examples/minimal.yml (the smallest useful Gmail config), examples/job-search.yml (an externally-managed filter file in anger), and examples/read_delivered.py (a correct consumer). All three configs pass mail-muncher validate --config <file>.

Acknowledgements

The wordmark is set in Silkscreen by Jason Kottke, used under the SIL Open Font License 1.1. Brand assets, the palette and usage rules are in branding/BRAND.md.

License

MIT. See LICENSE.