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.

!Resend Box Screenshot

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

$3

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

Installation

``bash
npx resend-box
`

Quick Start

1. Initialize your project (recommended):

`bash
npx resend-box init
`

Learn more

2. Start the sandbox:

`bash
npx resend-box start
`

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

Usage

$3

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

`typescript
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: '

Hello world!

',
})
`

Configuration

$3

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

$3

You can override ports via CLI flags or environment variables:

`bash


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
`

$3

`bash


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
`

$3

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.

$3

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

API Endpoints

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POST /emails

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

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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

$3

`typescript
// 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: '

Hello world!

',
})

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' },
})
})
`

$3

`typescript
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: '

Hello from SMTP!

',
})
`

$3

`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()
`

$3

`bash


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": "

Hello!

"
}'

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}
``

---

Built with coffee by Tomás Pozo