A local email sandbox + catcher for individual developers. Runs on your laptop, catches every email your app sends, and gives you a web sandbox to browse, search, and inspect them in. One self-contained binary. Open source. MIT-licensed.
- SMTP server on
127.0.0.1:3535— point any app at it; every message you send locally gets caught. - Web sandbox on
127.0.0.1:3550— search, view, inspect HTML/text/raw/headers/attachments. - REST API on
127.0.0.1:3550— JSON over HTTP; existing API clients only need a host override. - Validation tools: HTML email-client compatibility check across major mail clients.
- Workflow: categories, message release to production SMTP, outbound webhooks, sendmail-replacement binary mode.
Browse caught messages, preview rendered HTML, and inspect client compatibility — all offline, on your machine.
| Inbox + HTML preview | HTML Check | Getting started (code samples) |
|---|---|---|
 |
 |
 |
- Not a team tool. No user accounts, no authentication, no multi-tenancy, no shared sandboxes, no API tokens, no RBAC.
- Not infrastructure. No Postgres support, no high-availability, no Prometheus metrics, no Kubernetes probes. SQLite only. Localhost only.
- Not a Mailtrap Sandbox replacement. If you need team features and cloud setup, use Mailtrap — the hosted product this tool complements.
| Component | Technology |
|---|---|
| Single binary | Go 1.26+ — SMTP, HTTP/JSON API, embedded SPA, all in one process |
| SMTP ingest | emersion/go-smtp + jhillyerd/enmime |
| HTTP routing | go-chi/chi |
| Storage | SQLite via modernc.org/sqlite (pure Go — no CGo) |
| Web UI | React 19 + tailwindcss + Radix UI + Vite, embedded into the binary at build time via //go:embed |
| Realtime | Plain WebSocket (/cable) |
| Distribution | brew install, docker run, single static binary (macOS + Linux; Windows not yet) |
brew tap mailtrap/local
brew install mailtrap-local
brew services start mailtrap-localThen open http://127.0.0.1:3550 and point your app's SMTP client at 127.0.0.1:3535.
docker run --rm \
-p 3535:3535 -p 3550:3550 \
-v mailtrap-local:/var/lib/mailtrap-local \
ghcr.io/mailtrap/mailtrap-local:latestThe same image is mirrored to Docker Hub at mailtrap/mailtrap-local:latest.
Download the right archive for your platform from the Releases page, extract, and run:
tar xzf mailtrap-local_<version>_<os>_<arch>.tar.gz
./mailtrap-localOn macOS, a one-time Gatekeeper bypass is required because the binary isn't notarized:
xattr -d com.apple.quarantine ./mailtrap-localgit clone https://github.com/mailtrap/mailtrap-local && cd mailtrap-local
scripts/build.sh # builds frontend + Go binary → bin/mailtrap-local
bin/mailtrap-localgo install github.com/mailtrap/mailtrap-local/cmd/mailtrap-local@latest also works for Go developers, but installs the build-time placeholder UI instead of the real React SPA — Vite isn't part of go install. Use one of the channels above for the real UI.
bin/mailtrap-local listens on 127.0.0.1:3535 (SMTP) and 127.0.0.1:3550 (HTTP+UI+API) by default. Pass --http-listen, --smtp-listen, --db, or --version to override.
Symlink (or rename) the binary to sendmail and put it on $PATH:
ln -sf "$(brew --prefix)/bin/mailtrap-local" /usr/local/bin/sendmailPHP's mail(), cron, mailx, and anything else that shells out to sendmail -t -i will hand the message off to the local sandbox. The binary detects it was invoked as sendmail (or mailtrap-sendmail, or via mailtrap-local sendmail …) and switches modes automatically.
# Prereqs: Go 1.26+, Node 22+, foreman (optional, for bin/dev)
# Install via mise/asdf using .tool-versions, or directly.
bin/setup # go mod download + npm install
bin/dev # boots backend + Vite concurrently (Procfile.dev)In dev:
| Port | Served by | Purpose |
|---|---|---|
3535 |
go run ./cmd/mailtrap-local |
SMTP ingest |
3540 |
Vite dev | SPA — proxies /api + /cable to backend |
3550 |
Go binary | JSON API + WebSocket |
Open http://127.0.0.1:3540 for the dev UI (with HMR). The production single-binary serves the same UI at http://127.0.0.1:3550 out of the embedded dist/.
mailtrap-local/
├── cmd/mailtrap-local/ # main + sendmail dispatch + embed staging
├── internal/
│ ├── api/ # chi handlers, wire types, SPA serving
│ ├── smtpd/ # go-smtp wrapper + enmime ingest
│ ├── store/ # SQLite schema + queries
│ ├── jobs/ # cloud mirror / relay / webhook / retention
│ ├── htmlcheck/ # HTML email compatibility rule engine
│ ├── live/ # in-process pub-sub for WebSocket
│ ├── relay/ # outbound SMTP relay client
│ ├── webhook/ # outbound webhook signed-POST client
│ ├── cloud/ # Mailtrap sandbox API client
│ └── config/ # YAML config loader
├── frontend/ # React + Vite SPA (built into cmd/mailtrap-local/dist/)
├── docs/
│ ├── api/openapi.yaml # API spec (embedded + served at /api/v1/openapi.yaml)
│ └── images/ # README screenshots + demo GIF
├── scripts/build.sh # frontend build + go build → bin/mailtrap-local
├── Dockerfile # multi-stage source build (Node → Go → distroless)
├── Dockerfile.goreleaser # release-time wrapper (goreleaser-built binary → distroless)
├── .goreleaser.yaml # release pipeline: binaries + Homebrew tap + GHCR + Docker Hub
└── .github/workflows/ # ci.yml + release.yml
The Homebrew formula lives in a separate tap repo (mailtrap/homebrew-local); goreleaser rewrites it on every tag.
The full OpenAPI 3.1 specification is at docs/api/openapi.yaml. When the app is running, the same spec is served at http://127.0.0.1:3550/api/v1/openapi.yaml. A bare GET on /api/v1 redirects to the hosted docs at https://docs.mailtrap.io/.
Optional YAML config at $XDG_CONFIG_HOME/mailtrap-local/config.yml (or ~/.config/mailtrap-local/config.yml). Sections:
storage.max_messages— retention cap (oldest evicted past the limit).cloud— Mailtrap sandbox API token + sandbox ID for the "Send to Cloud" button.relay— outbound SMTP relay credentials for the "Release" button.webhook— outbound webhook URL + signing secret.
Values support ${ENV_VAR} interpolation. Keys present in the config file override the equivalent values stored in the SQLite DB; pinned fields are read-only in the UI.
go test ./...Unit + integration tests cover the store, API, SMTP server, jobs dispatcher, HTML check, and sendmail mode. The OpenAPI drift test (internal/api/openapi_drift_test.go) fails the build if a route is added to chi without speccing it (or vice versa).
MIT — see LICENSE.