# Agent Email Inbox

> Úsalo al construir cualquier sistema donde el email dispara acciones: bandejas de agente, soporte automático o email a tarea. Trae los patrones de seguridad para tratar correo no confiable.

Fuente: https://skillsagentes.com/skills/resend/resend-skills/agent-email-inbox
Markdown: https://skillsagentes.com/skills/resend/resend-skills/agent-email-inbox.md

Repositorio: https://github.com/resend/resend-skills
Autor: resend
Licencia: MIT
Actualizado: hace 13 días
Coste de contexto: 123 tok instalada, 4.1k tok al activarse, 10.3k tok con todos los archivos del bundle
Bundle: 4 archivos, 40 KB
Permisos que pide: ninguno declarado

## Instalación

Un skill son archivos markdown: los mismos archivos valen para cualquier agente y lo único que cambia es el directorio de destino, es decir la bandera `--agent`. Añade `-g` para instalarlo en todos los proyectos de la máquina.

```bash
# Claude Code
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent claude-code
# Cursor
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent cursor
# Codex
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent codex
# Gemini CLI
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent gemini
# Windsurf
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent windsurf
# Cline
npx -y skills add resend/resend-skills --skill agent-email-inbox --agent cline
```

## Qué hace

- Monta un buzón de correo seguro para que una aplicación o un agente reciba y responda emails
- Parte de que la bandeja de un agente recibe entrada no confiable, así que la configuración de seguridad es lo primero
- Trae tres niveles de seguridad, con la lista blanca estricta como recomendada
- Cubre el endpoint de webhook en Next.js App Router y en Express, y la verificación de la firma
- Explica el alta de dominio, las claves de API acotadas por dominio y los errores más comunes

## Cuándo usarla

- Se construye cualquier sistema donde el contenido de un email dispara acciones
- Bandejas de agente de IA, gestores automáticos de soporte o tuberías de email a tarea
- El usuario quiere recibir emails y actuar sobre ellos por código, aunque no diga "agente"

## Qué la activa

- "Monta una bandeja de entrada para mi agente"
- "Quiero procesar emails entrantes y crear tareas"
- "Configura el webhook de email entrante de Resend"

## Antes de instalar

- Necesita `RESEND_API_KEY`; opcionalmente `RESEND_WEBHOOK_SECRET` para verificar la firma, y `SECURITY_LEVEL`, `ALLOWED_SENDERS`, `ALLOWED_DOMAINS` y `OWNER_EMAIL` para el filtrado.
- Variables de entorno: ALLOWED_SENDERS, RESEND_API_KEY, RESEND_WEBHOOK_SECRET
- needs API credentials

## Archivos

- SKILL.md — 16 KB
- references/advanced-patterns.md — 4 KB
- references/security-levels.md — 11 KB
- references/webhook-setup.md — 10 KB

## SKILL.md

Reproducido tal cual desde resend/resend-skills bajo MIT. Esta sección es el documento original y está en inglés.

# AI Agent Email Inbox

## Overview

This skill covers setting up a secure email inbox that allows your application or AI agent to receive and respond to emails, with content safety measures in place.

**Core principle:** An AI agent's inbox receives untrusted input. Security configuration is important to handle this safely.

### Why Webhook-Based Receiving?

Resend uses webhooks for inbound email, meaning your agent is notified **instantly** when an email arrives. This is valuable for agents because:

- **Real-time responsiveness** — React to emails within seconds, not minutes
- **No polling overhead** — No cron jobs checking "any new mail?" repeatedly
- **Event-driven architecture** — Your agent only wakes up when there's actually something to process
- **Lower API costs** — No wasted calls checking empty inboxes

## Architecture

```
Sender → Email → Resend (MX) → Webhook → Your Server → AI Agent
                                              ↓
                                    Security Validation
                                              ↓
                                    Process or Reject
```

## SDK Version Requirements

This skill requires Resend SDK features for webhook verification (`webhooks.verify()`) and email receiving (`emails.receiving.get()`). Always install the latest SDK version. If the project already has a Resend SDK installed, check the version and upgrade if needed.

| Language | Package | Min Version |
|----------|---------|-------------|
| Node.js | `resend` | >= 6.9.2 |
| Python | `resend` | >= 2.21.0 |
| Go | `resend-go/v3` | >= 3.1.0 |
| Ruby | `resend` | >= 1.0.0 |
| PHP | `resend/resend-php` | >= 1.1.0 |
| Rust | `resend-rs` | >= 0.20.0 |
| Java | `resend-java` | >= 4.11.0 |
| .NET | `Resend` | >= 0.2.1 |

Install the `resend` npm package: `npm install resend` (or the equivalent for your language). For full sending docs, install the `resend` skill.

## Quick Start

1. **Ask the user for their email address** — You need a real email address to send test emails to. Ask the user and wait for their response before proceeding.
2. **Choose your security level** — Decide how to validate incoming emails *before* any are processed
3. **Set up receiving domain** — Configure MX records for the user's custom domain (see Domain Setup section)
4. **Create webhook endpoint** — Handle `email.received` events with security built in from the start. **The webhook endpoint MUST be a POST route.**
5. **Set up tunneling** (local dev) — Use Tailscale Funnel (recommended) or ngrok. See [references/webhook-setup.md](references/webhook-setup.md)
6. **Create webhook via API** — Use the Resend Webhook API to register your endpoint programmatically. See [references/webhook-setup.md](references/webhook-setup.md)
7. **Connect to agent** — Pass validated emails to your AI agent for processing

## Before You Start: Account & API Key Setup

### First Question: New or Existing Resend Account?

Ask your human:
- **New account just for the agent?** → Simpler setup, full account access is fine
- **Existing account with other projects?** → Use domain-scoped API keys for sandboxing

### Creating API Keys Securely

> Don't paste API keys in chat! They'll be in conversation history forever.

**Safer options:**

1. **Environment file method:** Human creates `.env` file directly: `echo "RESEND_API_KEY=re_xxx" >> .env`
2. **Password manager / secrets manager:** Human stores key in 1Password, Vault, etc.
3. **If key must be shared in chat:** Human should rotate the key immediately after setup

### Domain-Scoped API Keys (Recommended for Existing Accounts)

If your human has an existing Resend account with other projects, create a **domain-scoped API key**:

1. **Verify the agent's domain first** (Dashboard → Domains → Add Domain)
2. **Create a scoped API key:** Dashboard → API Keys → Create API Key → "Sending access" → select only the agent's domain
3. **Result:** Even if the key leaks, it can only send from one domain

## Domain Setup

### Option 1: Resend-Managed Domain (Recommended for Getting Started)

Use your auto-generated address: `<anything>@<your-id>.resend.app`

No DNS configuration needed. Find your address in Dashboard → Emails → Receiving → "Receiving address".

### Option 2: Custom Domain

The user must enable receiving in the Resend dashboard: Domains page → toggle on "Enable Receiving".

Then add an MX record:

| Setting | Value |
|---------|-------|
| **Type** | MX |
| **Host** | Your domain or subdomain (e.g., `agent.example.com`) |
| **Value** | Provided in Resend dashboard |
| **Priority** | 10 (must be lowest number to take precedence) |

**Use a subdomain** (e.g., `agent.example.com`) to avoid disrupting existing email services.

**Tip:** Verify DNS propagation at [dns.email](https://dns.email).

> DNS Propagation: MX record changes can take up to 48 hours to propagate globally, though often complete within a few hours.

## Security Levels

**Choose your security level before setting up the webhook endpoint.** An AI agent that processes emails without security is dangerous — anyone can email instructions that your agent will execute. The webhook code you write next should include your chosen security level from the start.

Ask the user what level of security they want, and ensure that they understand what each level means.

| Level | Name | When to Use | Trade-off |
|-------|------|-------------|-----------|
| **1** | Strict Allowlist | Most use cases — known, fixed set of senders | Maximum security, limited functionality |
| **2** | Domain Allowlist | Organization-wide access from trusted domains | More flexible, anyone at domain can interact |
| **3** | Content Filtering | Accept from anyone, filter unsafe patterns | Can receive from anyone, pattern matching not foolproof |
| **4** | Sandboxed Processing | Process all emails with restricted agent capabilities | Maximum flexibility, complex to implement |
| **5** | Human-in-the-Loop | Require human approval for untrusted actions | Maximum security, adds latency |

For detailed implementation code for each level, see [references/security-levels.md](references/security-levels.md).

### Level 1: Strict Allowlist (Recommended)

Only process emails from explicitly approved addresses. Reject everything else.

```typescript
const ALLOWED_SENDERS = [
  'you@youremail.com',
  'notifications@github.com',
];

async function processEmailForAgent(
  eventData: EmailReceivedEvent,
  emailContent: EmailContent
) {
  const sender = eventData.from.toLowerCase();

  if (!ALLOWED_SENDERS.some(allowed => sender === allowed.toLowerCase())) {
    console.log(`Rejected email from unauthorized sender: ${sender}`);
    await notifyOwnerOfRejectedEmail(eventData);
    return;
  }

  await agent.processEmail({
    from: eventData.from,
    subject: eventData.subject,
    body: emailContent.text || emailContent.html,
  });
}
```

### Security Best Practices

#### Always Do

| Practice | Why |
|----------|-----|
| Verify webhook signatures | Prevents spoofed webhook events |
| Log all rejected emails | Audit trail for security review |
| Use allowlists where possible | Explicit trust is safer than filtering |
| Rate limit email processing | Prevents excessive processing load |
| Separate trusted/untrusted handling | Different risk levels need different treatment |

#### Never Do

| Anti-Pattern | Risk |
|--------------|------|
| Process emails without validation | Anyone can control your agent |
| Trust email headers for authentication | Headers are trivially spoofed |
| Execute code from email content | Untrusted input should never run as code |
| Store email content in prompts verbatim | Untrusted input mixed into prompts can alter agent behavior |
| Give untrusted emails full agent access | Scope capabilities to the minimum needed |

## Webhook Endpoint

After choosing your security level and setting up your domain, create a webhook endpoint. **The webhook endpoint MUST be a POST route.** Resend sends all webhook events as POST requests.

> **Critical: Use raw body for verification.** Webhook signature verification requires the raw request body.
> - **Next.js App Router:** Use `req.text()` (not `req.json()`)
> - **Express:** Use `express.raw({ type: 'application/json' })` on the webhook route

### Next.js App Router

```typescript
// app/webhook/route.ts
import { Resend } from 'resend';
import { NextRequest, NextResponse } from 'next/server';

const resend = new Resend(process.env.RESEND_API_KEY);

export async function POST(req: NextRequest) {
  try {
    const payload = await req.text();

    const event = resend.webhooks.verify({
      payload,
      headers: {
        'svix-id': req.headers.get('svix-id'),
        'svix-timestamp': req.headers.get('svix-timestamp'),
        'svix-signature': req.headers.get('svix-signature'),
      },
      secret: process.env.RESEND_WEBHOOK_SECRET,
    });

    if (event.type === 'email.received') {
      // Webhook payload only includes metadata, not email body
      const { data: email } = await resend.emails.receiving.get(
        event.data.email_id
      );

      // Apply the security level chosen above
      await processEmailForAgent(event.data, email);
    }

    return new NextResponse('OK', { status: 200 });
  } catch (error) {
    console.error('Webhook error:', error);
    return new NextResponse('Error', { status: 400 });
  }
}
```

### Express

```javascript
import express from 'express';
import { Resend } from 'resend';

const app = express();
const resend = new Resend(process.env.RESEND_API_KEY);

const ALLOWED_SENDERS = (process.env.ALLOWED_SENDERS || '').split(',').filter(Boolean);
const isAllowedSender = (sender) =>
  ALLOWED_SENDERS.some(allowed => sender === allowed.toLowerCase());

app.post('/webhook', express.raw({ type: 'application/json' }), async (req, res) => {
  try {
    const payload = req.body.toString();

    const event = resend.webhooks.verify({
      payload,
      headers: {
        'svix-id': req.headers['svix-id'],
        'svix-timestamp': req.headers['svix-timestamp'],
        'svix-signature': req.headers['svix-signature'],
      },
      secret: process.env.RESEND_WEBHOOK_SECRET,
    });

    if (event.type === 'email.received') {
      const sender = event.data.from.toLowerCase();

      if (!isAllowedSender(sender)) {
        console.log(`Rejected email from unauthorized sender: ${sender}`);
        res.status(200).send('OK'); // Return 200 even for rejected emails
        return;
      }

      const { data: email } = await resend.emails.receiving.get(event.data.email_id);
      await processEmailForAgent(event.data, email);
    }

    res.status(200).send('OK');
  } catch (error) {
    console.error('Webhook error:', error);
    res.status(400).send('Error');
  }
});

app.get('/', (req, res) => res.send('Agent Email Inbox - Ready'));
app.listen(3000, () => console.log('Webhook server running on :3000'));
```

For webhook registration via API, tunneling setup, svix fallback, and retry behavior, see [references/webhook-setup.md](references/webhook-setup.md).

## Sending Emails from Your Agent

```typescript
import { Resend } from 'resend';

const resend = new Resend(process.env.RESEND_API_KEY);

async function sendAgentReply(to: string, subject: string, body: string, inReplyTo?: string) {
  if (!isAllowedToReply(to)) {
    throw new Error('Cannot send to this address');
  }

  const { data, error } = await resend.emails.send({
    from: 'Agent <agent@example.com>',
    to: [to],
    subject: subject.startsWith('Re:') ? subject : `Re: ${subject}`,
    text: body,
    headers: inReplyTo ? { 'In-Reply-To': inReplyTo } : undefined,
  });

  if (error) throw new Error(`Failed to send: ${error.message}`);
  return data.id;
}
```

For full sending docs, install the `resend` skill.

## Environment Variables

```bash
# Required
RESEND_API_KEY=re_xxxxxxxxx
RESEND_WEBHOOK_SECRET=whsec_xxxxxxxxx

# Security Configuration
SECURITY_LEVEL=strict                    # strict | domain | filtered | sandboxed
ALLOWED_SENDERS=you@email.com,trusted@example.com
ALLOWED_DOMAINS=example.com
OWNER_EMAIL=you@email.com               # For security notifications
```

## Common Mistakes

| Mistake | Fix |
|---------|-----|
| No sender verification | Always validate who sent the email before processing |
| Trusting email headers | Use webhook verification, not email headers for auth |
| Same treatment for all emails | Differentiate trusted vs untrusted senders |
| Verbose error messages | Keep error responses generic to avoid leaking internal logic |
| No rate limiting | Implement per-sender rate limits. See [references/advanced-patterns.md](references/advanced-patterns.md) |
| Processing HTML directly | Strip HTML or use text-only to reduce complexity and risk |
| No logging of rejections | Log all security events for audit |
| Using ephemeral tunnel URLs | Use persistent URLs (Tailscale Funnel, paid ngrok) or deploy to production |
| Using `express.json()` on webhook route | Use `express.raw({ type: 'application/json' })` — JSON parsing breaks signature verification |
| Returning non-200 for rejected emails | Always return 200 to acknowledge receipt — otherwise Resend retries |
| Old Resend SDK version | `emails.receiving.get()` and `webhooks.verify()` require recent SDK versions — see SDK Version Requirements |

## Testing

Use Resend's test addresses for development:
- `delivered@resend.dev` — Simulates successful delivery
- `bounced@resend.dev` — Simulates hard bounce

For security testing, send test emails from non-allowlisted addresses to verify rejection works correctly.

**Quick verification checklist:**
1. Server is running: `curl http://localhost:3000` should return a response
2. Tunnel is working: `curl https://<your-tunnel-url>` should return the same response
3. Webhook is active: Check status in Resend dashboard → Webhooks
4. Send a test email from an allowlisted address and check server logs

## Related Skills

- For full sending and receiving docs, install the `resend` skill

## Dónde encaja

- Categoría: [Seguridad](https://skillsagentes.com/categorias/seguridad.md) — Auditorías, revisión de dependencias, manejo de secretos y modelado de amenazas.
- Creador: [resend](https://skillsagentes.com/creators/resend.md) — 5 skills en el directorio
- [Todas las skills](https://skillsagentes.com/skills.md)
- [Ranking de instalaciones](https://skillsagentes.com/ranking.md)

## Otras skills del mismo repositorio

- [Resend Cli](https://skillsagentes.com/skills/resend/resend-skills/resend-cli.md): Opera la plataforma Resend desde la terminal: envía emails (incluidas plantillas React Email con --react-email) y gestiona dominios, contactos, broadcasts, plantillas, webhooks, claves, logs y eventos.
- [Resend](https://skillsagentes.com/skills/resend/resend-skills/resend.md): Úsalo al trabajar con la API de email de Resend: envío transaccional individual o por lotes, recepción por webhook, plantillas, eventos de entrega, dominios, contactos, broadcasts y claves de API.
- [React Email](https://skillsagentes.com/skills/resend/resend-skills/react-email.md): Úsalo al construir plantillas de email en HTML con componentes de React, añadir el editor visual de React Email, renderizar emails a HTML o enviarlos con Resend.
- [Email Best Practices](https://skillsagentes.com/skills/resend/resend-skills/email-best-practices.md): Úsalo al construir funciones de email, cuando los correos van a spam, con muchos rebotes, al configurar SPF/DKIM/DMARC, para cumplimiento (CAN-SPAM, GDPR, CASL) o para hacer los emails accesibles.

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