Resend Box

A local email sandbox that acts as both a Resend API mock and an SMTP server, with a modern web UI for inspecting captured emails.

Features

• 🎯 Resend API Compatible: Mock the Resend API for local development
• 🚀 Resend emails.send Compatible: Seamlessly send emails to your local sandbox using text, HTML, or React components via the Resend SDK.
• 📧 SMTP Server: Accept emails via SMTP protocol
• 🔍 Email Inspection: View HTML, text, and raw MIME content

Limitations

• The Resend API Mock only implements the POST /emails endpoint (resend.emails.send). Other Resend API endpoints are not supported.

Installation

npx resend-box

Quick Start

1. Initialize your project (recommended):

   npx resend-box init

Learn more

2. Start the sandbox:

   npx resend-box start

3. View captured emails: Open http://127.0.0.1:4657 in your browser.

Usage

Resend SDK

RESEND_BASE_URL will be loaded by the Resend SDK automatically. No changes needed to your code.

import { Resend } from 'resend'

const resend = new Resend('re_test_...')

const { data } = await resend.emails.send({
  from: 'you@example.com',
  to: 'user@gmail.com',
  subject: 'Hello',
  html: '<p>Hello world!</p>',
})

Configuration

Default Ports

• HTTP port (serves API and UI): 4657
• SMTP port: 1025

CLI Options

You can override ports via CLI flags or environment variables:

# Using CLI flags
npx resend-box start --http-port 3000 --smtp-port 2525

# Using environment variables
RESENDBOX_HTTP_PORT=3000 RESENDBOX_SMTP_PORT=2525 npx resend-box

CLI Commands

# Start the sandbox
npx resend-box start

# Initialize configuration in your project
npx resend-box init
npx resend-box init --base-url http://127.0.0.1:3000

# Use custom ports for init
npx resend-box start --http-port 3000 --smtp-port 2525

# Show help
npx resend-box --help

Init Command

The init command automatically:

• Configures .env.local or .env with:
  • RESEND_API_KEY (generates a demo key if missing)
  • RESEND_BASE_URL (points to local sandbox)
  • SMTP settings (SMTP_HOST, SMTP_PORT, etc.)
• Updates Supabase config.toml if a Supabase project is detected
• Detects project context:
  • host.docker.internal for Supabase/Docker projects
  • 127.0.0.1 for local development
• Shows a summary and asks for confirmation before making changes

Don't forget to restart your supabase project after updating your config.toml

Architecture

Resend Box consists of three main components:

1. Resend API Mock (/emails): Accepts POST requests matching the Resend API format and stores emails in memory
2. SMTP Server (port 1025): Accepts emails via SMTP protocol and normalizes them to the same format
3. Web API & UI (/sandbox/*): Provides REST endpoints for the UI and serves the React application

All emails are stored in an in-memory store that persists for the lifetime of the server process. The store normalizes emails from both sources (Resend API and SMTP) into a unified format which is accessible from the UI.

Data Flow

┌─────────────┐         ┌───────────────┐
│ Resend SDK  │────────▶│ Resend API    │
│             │         │ Mock (/emails)│
└─────────────┘         └───────┬───────┘
                                │
┌─────────────┐         ┌──────▼───────┐
│ SMTP Client │────────▶│ SMTP Server  │
│             │         │  (port 1025) │
└─────────────┘         └──────┬───────┘
                                │
                         ┌──────▼───────┐
                         │ Email Store  │
                         │  (in-memory) │
                         └──────┬───────┘
                                │
                         ┌──────▼───────┐
                         │  Web API     │
                         │ (/sandbox/*) │
                         └──────┬───────┘
                                │
                         ┌──────▼───────┐
                         │  React UI    │
                         │  (/ui/*)     │
                         └──────────────┘

API Endpoints

Resend API Mock

POST /emails

• Accepts Resend-compatible email payloads
• Returns: { id: string, created_at: string, to: string[], from: string }

Web API (for UI)

GET /sandbox/emails

• Returns: { emails: Email[] } (newest first)

GET /sandbox/emails/:id

• Returns: { email: Email } or 404 { error: string }

DELETE /sandbox/emails

• Clears all emails
• Returns: { message: string }

DELETE /sandbox/emails/:id

• Deletes a specific email
• Returns: { message: string } or 404 { error: string }

Usage Examples

Resend SDK from Supabase Edge Function

// supabase/functions/send-email/index.ts
import { Resend } from 'https://esm.sh/resend@3.0.0'
import { serve } from 'https://deno.land/std@0.168.0/http/server.ts'

serve(async (req) => {
  // After running 'resend-box init', RESEND_API_KEY and RESEND_BASE_URL are set
  const resend = new Resend(Deno.env.get('RESEND_API_KEY'))

  const { data, error } = await resend.emails.send({
    from: 'you@example.com',
    to: 'user@gmail.com',
    subject: 'Hello',
    html: '<p>Hello world!</p>',
  })

  if (error) {
    return new Response(JSON.stringify({ error }), {
      status: 400,
      headers: { 'Content-Type': 'application/json' },
    })
  }

  return new Response(JSON.stringify({ id: data?.id }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' },
  })
})

SMTP (Node.js with nodemailer)

import nodemailer from 'nodemailer'

const transporter = nodemailer.createTransport({
  host: process.env.SMTP_HOST || '127.0.0.1',
  port: parseInt(process.env.SMTP_PORT || '1025', 10),
  secure: false,
  auth: {
    user: process.env.SMTP_USER || 'admin',
    pass: process.env.SMTP_PASSWORD || 'admin',
  },
})

await transporter.sendMail({
  from: 'sender@example.com',
  to: 'recipient@example.com',
  subject: 'Test Email',
  text: 'Hello from SMTP!',
  html: '<p>Hello from SMTP!</p>',
})

SMTP (Python)

import os
import smtplib
from email.mime.text import MIMEText

msg = MIMEText('Hello from Python!')
msg['Subject'] = 'Test Email'
msg['From'] = 'sender@example.com'
msg['To'] = 'recipient@example.com'

host = os.getenv('SMTP_HOST', '127.0.0.1')
port = int(os.getenv('SMTP_PORT', '1025'))
user = os.getenv('SMTP_USER', 'admin')
password = os.getenv('SMTP_PASSWORD', 'admin')

server = smtplib.SMTP(host, port)
server.login(user, password)
server.send_message(msg)
server.quit()

Direct API Calls

# Send email via Resend API mock
curl -X POST http://127.0.0.1:4657/emails \
  -H "Content-Type: application/json" \
  -d '{
    "from": "test@example.com",
    "to": "user@example.com",
    "subject": "Test Email",
    "html": "<p>Hello!</p>"
  }'

# List all emails
curl http://127.0.0.1:4657/sandbox/emails

# Get specific email
curl http://127.0.0.1:4657/sandbox/emails/{email-id}

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Built with coffee by Tomás Pozo