imapsync-go

imapsync-go is a lightweight Go CLI that mirrors folders between two IMAP accounts. It builds a sync plan from message IDs, streams mail bodies directly between servers.

Note: This project was created in collaboration with the Claude Code.

Motivation

This tool was built to provide a simple, fast, and dependency-free solution for IMAP synchronization. Key goals:

• Zero dependencies - Single static binary with no external requirements
• Cross-platform - Works on Linux and macOS (amd64/arm64)
• Efficient - Parallel workers for fast synchronization
• Simple - Easy configuration via JSON or YAML

Unlike heavyweight alternatives, imapsync-go focuses on doing one thing well: efficiently copying IMAP folders between servers.

Installation

Homebrew (macOS)

brew tap greeddj/tap
brew install imapsync-go

Docker

Pull the image from GitHub Container Registry:

# Latest version
podman pull ghcr.io/greeddj/imapsync-go:latest
# or
podman pull ghcr.io/greeddj/imapsync-go:1.0.0

Binary Release

Download pre-built binaries from GitHub Releases:

# Example for Linux amd64
curl -LO https://github.com/greeddj/imapsync-go/releases/latest/download/imapsync-go_<version>_Linux_x86_64.tar.gz
tar xzf imapsync-go_<version>_Linux_x86_64.tar.gz
chmod +x imapsync-go
sudo mv imapsync-go /usr/local/bin/

Usage

Configuration

Create a configuration file (config.json or config.yaml):

JSON example:

{
  "src": {
    "label": "Source",
    "server": "imap.source.com:993",
    "user": "user@source.com",
    "pass": "password",
    "auth": "login"
  },
  "dst": {
    "label": "Destination",
    "server": "imap.dest.com:993",
    "user": "user@dest.com",
    "pass": "password",
    "auth": "cram-md5"
  },
  "map": [
    { "src": "INBOX", "dst": "INBOX" },
    { "src": "Sent", "dst": "Sent Items" }
  ]
}

YAML example:

src:
  label: Source
  server: imap.source.com:993
  user: user@source.com
  pass: password
  auth: login

dst:
  label: Destination
  server: imap.dest.com:993
  user: user@dest.com
  pass: password
  auth: cram-md5

map:
  - src: INBOX
    dst: INBOX
  - src: Sent
    dst: Sent Items

Note: The map section is optional. If omitted, imapsync-go automatically connects to the source server, lists all mailboxes, and builds a 1:1 mapping of every source folder to the same-named destination folder. You can also use --src-folder / --dest-folder to sync a single folder without a config file map section.

Authentication

The auth field in src or dst blocks is optional and specifies the authentication mechanism:

• login (default): Standard IMAP LOGIN authentication.
• cram-md5: CRAM-MD5 Challenge-Response authentication.

If omitted or set to an empty string, login is used.

Running with Homebrew

export IMAPSYNC_CONFIG="/Users/$(whoami)/.imapsync/prod_config.json"

# Show available folders
imapsync-go show

# Sync all folders (from config map or auto-discovered from source)
imapsync-go sync -w 4

# Sync a single folder
imapsync-go sync -s INBOX -d INBOX
imapsync-go sync -s 'Test.[some_group].box' -d 'Test/some_group/box'

# Auto-confirm without prompt
imapsync-go sync -y

Running with Docker

# show folders
podman run --rm \
-v /Users/$(whoami)/.imapsync/prod_config.json:/config.json ghcr.io/greeddj/imapsync-go:latest \
-c /config.json \
show

# sync folders
podman run --rm -it \
-v /Users/$(whoami)/.imapsync/prod_config.json:/config.json ghcr.io/greeddj/imapsync-go:latest \
-c /config.json \
sync -w 4

Command-line Options

Global flags:

• -c, --config - Path to configuration file (default: config.json)

Show command:

• -V, --verbose - Show additional detail (env: IMAPSYNC_VERBOSE)
• -q, --quiet - Suppress progress bars; output is plain text suitable for piping (env: IMAPSYNC_QUIET)

Sync command:

• -s, --src-folder - Source folder (overrides config) (env: IMAPSYNC_SOURCE_FOLDER)
• -d, --dest-folder - Destination folder (overrides config) (env: IMAPSYNC_DESTINATION_FOLDER)
• -w, --workers - Number of parallel workers (default: 4, max: 10) (env: IMAPSYNC_WORKERS)
• -y, --confirm, --yes - Auto-confirm without prompt (env: IMAPSYNC_CONFIRM)
• -V, --verbose - Enable verbose output (env: IMAPSYNC_VERBOSE)
• -q, --quiet - Suppress non-error output (env: IMAPSYNC_QUIET)
• --bps-down - Max bytes/sec read from the source server (0 = unlimited) (env: IMAPSYNC_BPS_DOWN)
• --bps-up - Max bytes/sec written to the destination server (0 = unlimited) (env: IMAPSYNC_BPS_UP)
• --max-connections - Hard cap on simultaneous IMAP connections per side (0 = no cap). One slot is reserved for the planning client, so --max-connections=N allows at most N−1 sync workers. (env: IMAPSYNC_MAX_CONNECTIONS)

The same bps-down, bps-up, and max-connections values can be set in config under a rate_limit block (down_bps, up_bps, max_connections). CLI flags take precedence when both are set.

Provider quotas (Gmail)

When either side is imap.gmail.com, imapsync-go prints a warning before the confirm prompt with the relevant Workspace IMAP limits:

• 15 simultaneous IMAP connections per account
• 2,500 MB/day download, 500 MB/day upload via IMAP
• Exceeding the bandwidth limit suspends the account for 1–24 hours

For Gmail, run with explicit throttling so the daily quotas don't trigger an account lockout mid-migration:

imapsync-go sync \
  --bps-down 300000 \
  --bps-up 300000 \
  --max-connections 10 \
  -w 9

--max-connections 10 reserves one slot for the planning client, leaving 9 slots for workers. -w 9 matches that ceiling; setting -w higher than maxConn−1 is harmless — the cap is enforced automatically.

The defaults (no throttle) match imapsync's default behaviour — they're appropriate for self-hosted IMAP servers on a LAN, not for big-provider mailboxes.

Notes

• Ctrl-C exits with code 130 and prints Cancelled. — this is the standard Unix convention for SIGINT termination and makes it composable in shell scripts.
• --version on source builds makes one HTTPS request to the GitHub releases API to fetch the latest tag. Set IMAPSYNC_OFFLINE=1 or run inside a CI environment (any non-empty CI variable) to skip that call and fall back to the build-time default.

Idempotency caveats

imapsync-go decides what to copy by diffing message Message-Id headers between source and destination. This is fast and re-running a partial sync is safe, but it has two known limitations:

• Messages without a Message-Id are skipped. Drafts, some bulk mail and messages from broken senders may not have one. They cannot be tracked across servers, so they are never copied. The CLI prints a warning per folder when this happens so you know how many were skipped.
• Servers that rewrite Message-Id on APPEND will cause duplicates on re-run. A few IMAP servers (notably some Exchange configurations) replace the inbound Message-Id with their own value. The diff on the next run will see the original IDs as still "missing" on the destination and re-upload them. If you suspect this, do the sync in a single pass and avoid re-running it against the same destination.

Known issues

Ctrl+C may not exit immediately during a network disruption

During a sync, if the network connection drops mid-transfer, pressing Ctrl+C may take up to a few minutes to fully cancel. The underlying IMAP client cannot interrupt a transfer that is waiting on a server response that never arrives (because the server is unreachable). TCP keepalive will eventually detect the dead connection — typically within a few minutes on macOS and Linux — and the program will exit cleanly once that happens.

If you need to abort immediately, press Ctrl+\ instead. This dumps all goroutine stacks to stderr and exits right away; it is the escape hatch for when the program appears stuck.

License

MIT