m2m

CLI to convert filesystem-based mailboxes, written in Rust.

Caution

m2m is in active development and shipped as v0.x.x. Expect breaking changes between releases until stabilisation.

Table of contents

• Features
• Installation
  • Cargo
  • Nix
  • Sources
• Usage
• Quirks
• Social
• Sponsoring

Features

• Convert from/to Maildir ^specs, Maildir++ ^specs and m2dir ^specs filesystem stores.
• Parallel copy loop sized to --workers (default = available parallelism, capped at 16).
• Idempotent re-runs via --skip-existing, which dedup-matches by Message-ID then by source id.
• Dry-run mode (--dry-run) lists what would be touched without creating or modifying any file.
• Inbox detection via --inbox <NAME> (default INBOX): case-insensitively identifies the source inbox folder and renames it to the canonical alias on the target. Maildir++ root surfaces under the same alias.
• Custom keyword round-trip between Maildir-family stores via two configurable side channels: the per-folder dovecot-keywords slot file (Dovecot / mbsync convention) and a body header (X-Keywords or X-Label).
• Header stripping on read (--strip-headers) to drop legacy markers (X-Mozilla-Status, X-Status, ...).
• Maildir++ layout awareness: the maildir++ subcommand reads / writes dotted folder siblings (.Work.Foo/) and reports them as logical Work/Foo mailboxes.
• Dovecot fs-layout via --fs-layout on the maildir subcommand: nested subfolders (Work/Foo/) instead of flat dotted siblings.

Tip

m2m is written in Rust and uses cargo features to gate backend support. The default feature set is declared in Cargo.toml.

Installation

Cargo

cargo install --locked --git https://github.com/pimalaya/m2m.git

Nix

If you have the Flakes feature enabled:

nix profile install github:pimalaya/m2m

Or run without installing:

nix run github:pimalaya/m2m

Sources

git clone https://github.com/pimalaya/m2m
cd m2m
nix run

Usage

The grammar is a chain of two subcommands, one per side, each carrying its own positional <PATH> and per-format flags:

m2m [SHARED-FLAGS] <SOURCE-CMD> <PATH> [SOURCE-FLAGS] <TARGET-CMD> <PATH> [TARGET-FLAGS]

Backends accepted as both source and target: maildir, maildir++, m2dir.

Examples:

m2m maildir ~/Mail m2dir ~/.Mail/example
m2m --workers 8 --skip-existing maildir ~/Mail --dovecot-keywords --keywords-header x-keywords m2dir ~/.Mail
m2m m2dir ~/.Mail maildir++ ~/Maildir
m2m --dry-run maildir ~/Mail m2dir /mnt/backup

Shared flags

Flag	Purpose
-w, --workers <N>	Worker threads (default = available parallelism, capped at 16).
--inbox <NAME>	Canonical inbox folder name (default INBOX); detected case-insensitively on the source, used as-is on the target.
--include-mailbox <NAME>	Process only these mailboxes (repeatable, case-insensitive).
--exclude-mailbox <NAME>	Skip these mailboxes (repeatable, case-insensitive).
--skip-existing	Skip messages already present on the target.
--dry-run	List the conversion plan without writing anything.
--json	Machine output instead of TTY spinner.
--log-level <off,error,warn,info,debug,trace>	Filter log output.

Quirks

maildir accepts four optional flags on either side, maildir++ accepts three (no --fs-layout), m2dir accepts none.

Flag	What it does
--dovecot-keywords	Reads / writes the per-folder dovecot-keywords sidecar (Dovecot / mbsync convention). Custom keywords in filenames (a..z) round-trip through the slot table.
--keywords-header <x-keywords|x-label>	Mirrors custom keywords into the chosen body header. x-keywords follows the OfflineIMAP / mbsync convention (comma-separated); x-label follows the mutt / notmuch convention (space-separated).
--strip-headers <CSV>	Drops these header names from each message on read; useful to clean Mozilla / Pegasus markers (X-Mozilla-Status, X-Status, ...).
--fs-layout	Dovecot fs layout: subfolders are nested filesystem directories (Work/Foo/) instead of flat dotted siblings. Only on the maildir subcommand.

When both sides set --dovecot-keywords and the dovecot slot table grows past 26 entries, the 27th keyword and onwards are dropped with a warn! log line per message; the conversion continues.

Social

• Chat on Matrix
• News on Mastodon or RSS
• Mail at pimalaya.org@posteo.net

Sponsoring

Special thanks to the NLnet foundation and the European Commission that have been financially supporting the project for years:

• 2022 → 2023: NGI Assure
• 2023 → 2024: NGI Zero Entrust
• 2024 → 2026: NGI Zero Core
• 2027 in preparation…

If you appreciate the project, feel free to donate using one of the following providers: