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MessagePit

An email and SMS testing tool for developers. Send test emails and SMS messages from your application and inspect them in a clean web UI — nothing reaches real inboxes or real phones.

MessagePit is a fork of Mailpit extended with a Twilio-compatible SMS ingest endpoint, a SendGrid v3 Mail Send API stub, SMS/email delivery status callbacks, an SMS inbox UI, and a dedicated SMS ingest server.

Features

  • Email: SMTP server, SendGrid v3 API stub, web UI, REST API, WebSocket live updates, search, tagging, POP3 server
  • SMS: Twilio-compatible HTTP ingest, SMS inbox with read/unread tracking, live WebSocket updates
  • Webhook capture: Dedicated HTTP server that captures any incoming request on any path/method and displays it in the UI — useful for inspecting outbound webhook calls from your app in development
  • Delivery callbacks: Signed SMS status callbacks and SendGrid-style email event webhooks for end-to-end delivery tracking
  • Shared: Multi-arch Docker image, optional HTTP basic auth, Prometheus metrics

Ports

Port Protocol Purpose
1025 SMTP Email ingest (mirrors port 25)
1110 POP3 POP3 server (optional)
1775 HTTP SMS ingest — Twilio-compatible (mirrors SMPP port 2775)
8025 HTTP Web UI, management API, and SendGrid v3 /v3/mail/send stub
8026 HTTP Webhook capture — accepts any request on any path/method

Quick Start

# Docker
docker run -p 1025:1025 -p 1775:1775 -p 8025:8025 -p 8026:8026 ghcr.io/coreydaley/messagepit

# From source
make run

Open http://localhost:8025 in your browser.

SMS Integration

Point your application's Twilio SDK at the SMS ingest server instead of api.twilio.com:

http://localhost:1775

The SMS server implements the Twilio Messages API endpoint:

POST /2010-04-01/Accounts/{AccountSid}/Messages.json

Required form fields: From, To, Body. Authentication uses HTTP Basic Auth (AccountSid:AuthToken).

SMS delivery callbacks

When MP_SMS_WEBHOOK_URL is set (or a per-request StatusCallback form field is provided), MessagePit fires a signed POST to that URL after capturing each message — mirroring how Twilio notifies your app of delivery status.

The callback body is application/x-www-form-urlencoded with MessageSid, MessageStatus, To, and From. When MP_SMS_AUTH_TOKEN is set the request includes an X-Twilio-Signature HMAC-SHA1 header so your webhook handler can validate it with the standard Twilio SDK.

Priority: the StatusCallback field in the send request takes precedence over the global MP_SMS_WEBHOOK_URL.

Webhook Capture

MessagePit runs a second HTTP server (port 8026 by default) that accepts any incoming HTTP request on any path and method, stores it, and displays it in the Webhooks tab of the UI. This is useful for developing and testing outbound webhook delivery from your application without needing a public endpoint.

Point your webhook URL at the capture server:

http://localhost:8026/any/path/you/like

Any HTTP method works: POST, GET, PUT, PATCH, DELETE. The full request — method, path, headers, body, source IP — is captured and displayed in real time via WebSocket.

The capture server is enabled by default. Set --webhook "" (or MP_WEBHOOK_BIND_ADDR="") to disable it.

Webhook capture API

Method Path Description
GET /api/v1/webhooks List captured requests (paginated)
GET /api/v1/webhooks/search Search captured requests
GET /api/v1/webhook/{id} Get a single captured request
DELETE /api/v1/webhook/{id} Delete a single captured request
DELETE /api/v1/webhooks Delete all captured requests

Email Integration (SendGrid v3)

MessagePit exposes a SendGrid v3 Mail Send stub:

POST /v3/mail/send
Authorization: Bearer <MP_SENDGRID_API_KEY>
Content-Type: application/json

The endpoint accepts the standard SendGrid v3 JSON payload (personalizations, from, subject, content, custom_args) and stores each message in the MessagePit mailbox. Authentication is skipped when MP_SENDGRID_API_KEY is empty.

Email delivery webhooks

When MP_EMAIL_WEBHOOK_URL is set, MessagePit fires a SendGrid-style event webhook after capturing each email that contains a notification_id key in custom_args. The webhook payload is a JSON array of event objects:

[
  {
    "notification_id": "<value from custom_args>",
    "event": "delivered",
    "email": "recipient@example.com",
    "timestamp": 1714000000
  }
]

Webhooks are signed using ECDSA P-256 / SHA-256, with the signature and timestamp in the same headers real SendGrid uses:

Header Description
X-Twilio-Email-Event-Webhook-Signature Base64-encoded ECDSA signature
X-Twilio-Email-Event-Webhook-Timestamp Unix timestamp as a string

The payload that is signed is timestamp + body (timestamp string concatenated with the raw JSON body).

Key pair setup

Provide a stable key pair so the public key doesn't change across restarts:

# Generate private key (SEC1 DER, base64) — set as MP_EMAIL_WEBHOOK_SIGNING_KEY
openssl ecparam -name prime256v1 -genkey -noout \
  | openssl ec -outform DER 2>/dev/null \
  | base64

# Derive the matching public key (PKIX DER, base64) — set as SENDGRID_WEBHOOK_PUBLIC_KEY in your app
openssl ecparam -name prime256v1 -genkey -noout \
  | openssl ec -outform DER 2>/dev/null \
  | openssl ec -inform DER -pubout -outform DER 2>/dev/null \
  | base64

If MP_EMAIL_WEBHOOK_SIGNING_KEY is empty and MP_EMAIL_WEBHOOK_URL is set, MessagePit auto-generates a key pair at startup and logs the public key — useful for one-off testing but not stable across restarts.

SMTP vs. SendGrid v3

The email webhook only fires for messages received via the /v3/mail/send endpoint, since only that path carries custom_args. Emails delivered over SMTP do not carry custom_args and will not trigger the webhook.

Building

Requires Go 1.21+ and Node 22+.

make run     # build UI + binary and run with dev defaults
make install # build UI + binary and install to $GOPATH/bin
make test    # run Go test suite
make ui      # build frontend assets only
make build   # compile the binary only (requires ui assets)

Configuration

All flags can also be set via environment variables (e.g. --smtp → MP_SMTP_BIND_ADDR, --sms → MP_SMS_BIND_ADDR).

Flag Env var Default Description
--smtp MP_SMTP_BIND_ADDR 0.0.0.0:1025 SMTP bind address
--sms MP_SMS_BIND_ADDR 0.0.0.0:1775 SMS ingest bind address
--listen MP_UI_BIND_ADDR 0.0.0.0:8025 HTTP UI/API bind address
--webhook MP_WEBHOOK_BIND_ADDR [::]:8026 HTTP webhook capture bind address (empty to disable)
--db MP_DATABASE (in-memory) SQLite database file path
--sms-auth-token MP_SMS_AUTH_TOKEN Twilio auth token — validates Basic Auth on inbound SMS; signs outgoing delivery callbacks
--sms-webhook-url MP_SMS_WEBHOOK_URL URL to POST SMS delivery callbacks to (fallback when no per-request StatusCallback)
--sendgrid-api-key MP_SENDGRID_API_KEY Expected Bearer token for /v3/mail/send (skipped when empty)
--email-webhook-url MP_EMAIL_WEBHOOK_URL URL to POST email delivery event webhooks to
--email-webhook-signing-key MP_EMAIL_WEBHOOK_SIGNING_KEY Base64-encoded SEC1 DER ECDSA P-256 private key (auto-generated when empty)

Run messagepit --help for the full list.

API

The REST API is documented at http://localhost:8025/api/v1.

SMS endpoints:

Method Path Description
GET /api/v1/sms/messages List SMS messages
GET /api/v1/sms/message/{id} Get single SMS message
PUT /api/v1/sms/message/{id}/read Mark as read
DELETE /api/v1/sms/message/{id} Delete message
DELETE /api/v1/sms/messages Delete all messages

Docker

Images are published to the GitHub Container Registry on every push to main and on tagged releases:

docker pull ghcr.io/coreydaley/messagepit:latest

docker-compose example

The example below enables full delivery callback support for both SMS and email. The extra_hosts entry is required on Linux so host.docker.internal resolves to the host machine (Docker Desktop handles this automatically on Mac and Windows).

services:
  messagepit:
    image: ghcr.io/coreydaley/messagepit:latest
    restart: always
    extra_hosts:
      - "host.docker.internal:host-gateway"
    ports:
      - "1025:1025"   # SMTP
      - "1775:1775"   # SMS ingest (Twilio-compatible)
      - "8025:8025"   # Web UI + SendGrid v3 stub
      - "8026:8026"   # Webhook capture
    environment:
      # SMS — must match TWILIO_AUTH_TOKEN in your app
      MP_SMS_AUTH_TOKEN: your-twilio-auth-token
      # Fallback SMS callback URL (per-request StatusCallback takes priority)
      MP_SMS_WEBHOOK_URL: http://host.docker.internal:3000/webhooks/v1/sms

      # SendGrid v3 stub — must match SENDGRID_API_KEY in your app
      MP_SENDGRID_API_KEY: your-sendgrid-api-key
      # Email delivery webhook URL
      MP_EMAIL_WEBHOOK_URL: http://host.docker.internal:3000/webhooks/v1/email
      # ECDSA private key for signing email webhooks (generate with openssl, see above)
      MP_EMAIL_WEBHOOK_SIGNING_KEY: "<base64-encoded SEC1 DER private key>"
    healthcheck:
      test: ["CMD", "/messagepit", "readyz"]
      interval: 10s
      timeout: 5s
      retries: 5

Rails Integration

MessagePit is designed to be a drop-in local replacement for Twilio and SendGrid, using the same official gems your production app uses.

Gems

# Gemfile
gem "twilio-ruby", "~> 7"
gem "sendgrid-actionmailer"

Environment variables (.env.development)

# SMS
TWILIO_ACCOUNT_SID=test
TWILIO_AUTH_TOKEN=test           # must match MP_SMS_AUTH_TOKEN
TWILIO_ACCOUNT_NUMBER=+15550000000
TWILIO_API_URL=http://localhost:1775
# Callback URL must use host.docker.internal so MessagePit (in Docker)
# can reach your Rails app running on the host.
TWILIO_STATUS_CALLBACK_URL=http://host.docker.internal:3000/webhooks/v1/sms

# Email
SENDGRID_API_KEY=test            # must match MP_SENDGRID_API_KEY
SENDGRID_API_URL=http://localhost:8025
# PKIX DER base64 public key matching MP_EMAIL_WEBHOOK_SIGNING_KEY
SENDGRID_WEBHOOK_PUBLIC_KEY=<base64-encoded PKIX DER public key>

Action Mailer (config/environments/development.rb)

config.action_mailer.delivery_method = :sendgrid_actionmailer
config.action_mailer.sendgrid_actionmailer_settings = {
  api_key:               ENV.fetch("SENDGRID_API_KEY", "test"),
  host:                  ENV.fetch("SENDGRID_API_URL", "https://api.sendgrid.com"),
  raise_delivery_errors: true
}

SMS sender

Route twilio-ruby to MessagePit by rewriting the API base URL. When TWILIO_API_URL is not set or equals https://api.twilio.com the real Twilio API is used.

require "twilio-ruby"

class SmsSender
  def self.call(to:, body:)
    sid      = ENV.fetch("TWILIO_ACCOUNT_SID")
    token    = ENV.fetch("TWILIO_AUTH_TOKEN")
    from     = ENV.fetch("TWILIO_ACCOUNT_NUMBER")
    callback = ENV["TWILIO_STATUS_CALLBACK_URL"]

    client = build_client(sid, token)
    params = { body: body, from: from, to: to }
    params[:status_callback] = callback if callback.present?

    message = client.messages.create(**params)
    { status: "Sent", code: 201, sid: message.sid }
  end

  def self.build_client(sid, token)
    base_url = ENV.fetch("TWILIO_API_URL", "https://api.twilio.com")
    return Twilio::REST::Client.new(sid, token) if base_url == "https://api.twilio.com"

    Twilio::REST::Client.new(sid, token, nil, nil, ProxyHttpClient.new(base_url))
  end

  # Rewrites the Twilio API host so the gem can target MessagePit.
  class ProxyHttpClient < Twilio::HTTP::Client
    def initialize(base_url)
      super()
      uri         = URI.parse(base_url)
      @proxy_host = "#{uri.scheme}://#{uri.host}"
      @proxy_base = "#{uri.scheme}://#{uri.host}:#{uri.port}"
      @proxy_port = uri.port
    end

    def request(host, port, method, url, params = {}, data = {}, headers = {}, auth = nil, timeout = nil)
      rewritten_url = url.sub(%r{\Ahttps?://[^/]+}, @proxy_base)
      super(@proxy_host, @proxy_port, method, rewritten_url, params, data, headers, auth, timeout)
    end
  end
end

Email delivery tracking

Set custom_args: { notification_id: record.id.to_s } in your mail() call. MessagePit extracts this value and includes it in the webhook payload so your app can update the delivery status on the corresponding record.

mail(
  to:          recipient,
  subject:     "Your subject",
  custom_args: { notification_id: @notification.id.to_s }
)

Webhook controllers

SMS — validate with Twilio::Security::RequestValidator:

require "twilio-ruby"

class SmsWebhookController < ApplicationController
  skip_before_action :verify_authenticity_token

  before_action :verify_twilio_signature

  def update
    notification = Notification.find_by(sms_id: params[:MessageSid])
    notification&.update_columns(sms_delivery_status: params[:MessageStatus])
    head :ok
  end

  private

  def verify_twilio_signature
    token     = ENV["TWILIO_AUTH_TOKEN"]
    validator = Twilio::Security::RequestValidator.new(token)
    unless validator.validate(request.original_url, request.POST, request.headers["X-Twilio-Signature"].to_s)
      head :forbidden
    end
  end
end

Email — validate with ECDSA using the SENDGRID_WEBHOOK_PUBLIC_KEY:

class EmailWebhookController < ApplicationController
  skip_before_action :verify_authenticity_token

  before_action :verify_sendgrid_signature

  def update
    (params["_json"] || []).each do |event|
      next unless event["event"] == "delivered" && event["notification_id"].present?
      Notification.find_by(id: event["notification_id"])
                  &.update_columns(email_delivery_status: "delivered")
    end
    head :ok
  end

  private

  def verify_sendgrid_signature
    public_key = OpenSSL::PKey.read(Base64.decode64(ENV["SENDGRID_WEBHOOK_PUBLIC_KEY"]))
    signature  = Base64.decode64(request.headers["X-Twilio-Email-Event-Webhook-Signature"].to_s)
    payload    = request.headers["X-Twilio-Email-Event-Webhook-Timestamp"].to_s + request.raw_post
    head :forbidden unless public_key.verify(OpenSSL::Digest::SHA256.new, signature, payload)
  rescue OpenSSL::PKey::PKeyError, ArgumentError
    head :forbidden
  end
end

License

MIT — see LICENSE.

Portions of this project are derived from Mailpit by Ralph Slooten, also MIT licensed.

About

An email and SMS testing tool for developers — fork of Mailpit with Twilio-compatible SMS ingest

github.com/coreydaley/messagepit

Topics

testing go twilio email sms smtp developer-tools mailpit

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Releases 4

v1.2.1 Latest
May 1, 2026
+ 3 releases

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