resend

Resend

Quick Send — Node.js

import { Resend } from 'resend';

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

const { data, error } = await resend.emails.send(
  {
    from: 'Acme <onboarding@resend.dev>',
    to: ['delivered@resend.dev'],
    subject: 'Hello World',
    html: '<p>Email body here</p>',
  },
  { idempotencyKey: `welcome-email/${userId}` }
);

if (error) {
  console.error('Failed:', error.message);
  return;
}
console.log('Sent:', data.id);

Key gotcha: The Resend Node.js SDK does NOT throw exceptions — it returns { data, error }. Always check error explicitly instead of using try/catch for API errors.

Quick Send — Python

import resend
import os

resend.api_key = os.environ["RESEND_API_KEY"]

email = resend.Emails.send({
    "from": "Acme <onboarding@resend.dev>",
    "to": ["delivered@resend.dev"],
    "subject": "Hello World",
    "html": "<p>Email body here</p>",
}, idempotency_key=f"welcome-email/{user_id}")

Single vs Batch Decision

Choose	When
Single (POST /emails)	1 email, needs attachments, needs scheduling
Batch (POST /emails/batch)	2-100 distinct emails, no attachments, no scheduling

Batch is atomic — if one email fails validation, the entire batch fails. Always validate before sending. Batch does NOT support attachments or scheduled_at.

Idempotency Keys (Critical for Retries)

Prevent duplicate emails when retrying failed requests:

Key Facts
Format (single)	<event-type>/<entity-id> (e.g., welcome-email/user-123)
Format (batch)	batch-<event-type>/<batch-id> (e.g., batch-orders/batch-456)
Expiration	24 hours
Max length	256 characters
Same key + same payload	Returns original response without resending
Same key + different payload	Returns 409 error

Quick Receive (Node.js)

import { Resend } from 'resend';

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

export async function POST(req: Request) {
  const payload = await req.text(); // Must use raw text, not req.json()

  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 has metadata only — call API for body
    const { data: email } = await resend.emails.receiving.get(
      event.data.email_id
    );
    console.log(email.text);
  }

  return new Response('OK', { status: 200 });
}

Key gotcha: Webhook payloads do NOT contain the email body. You must call resend.emails.receiving.get() separately.

What Do You Need?

Task	Reference
Send a single email	sending/overview.md — parameters, deliverability, testing
Send batch emails	sending/overview.md → sending/batch-email-examples.md
Full SDK examples (Node.js, Python, Go, cURL)	sending/single-email-examples.md
Idempotency, retries, error handling	sending/best-practices.md
Receive inbound emails	receiving.md — domain setup, webhooks, attachments
Manage templates (CRUD, variables)	templates.md — lifecycle, aliases, pagination
Set up webhooks (all event types)	webhooks.md — verification, retry schedule, IP allowlist
Install SDK (8+ languages)	installation.md
Set up an AI agent inbox	Install the agent-email-inbox skill — covers security levels for untrusted input
Marketing emails / newsletters	Use Resend Broadcasts — not batch sending

SDK Version Requirements

Always install the latest SDK version. These are the minimum versions for full functionality (sending, receiving, webhook verification):

Language	Package	Min Version	Install
Node.js	resend	>= 6.9.2	npm install resend
Python	resend	>= 2.21.0	pip install resend
Go	resend-go/v3	>= 3.1.0	go get github.com/resend/resend-go/v3
Ruby	resend	>= 1.0.0	gem install resend
PHP	resend/resend-php	>= 1.1.0	composer require resend/resend-php
Rust	resend-rs	>= 0.20.0	cargo add resend-rs
Java	resend-java	>= 4.11.0	See installation.md
.NET	Resend	>= 0.2.1	dotnet add package Resend

If the project already has a Resend SDK installed, check the version and upgrade if it's below the minimum. Older SDKs may be missing webhooks.verify() or emails.receiving.get().

See installation.md for full installation commands, language detection, and cURL fallback.

Common Setup

API Key

Store in environment variable — never hardcode:

export RESEND_API_KEY=re_xxxxxxxxx

Get your key at resend.com/api-keys.

Detect Project Language

Check for these files: package.json (Node.js), requirements.txt/pyproject.toml (Python), go.mod (Go), Gemfile (Ruby), composer.json (PHP), Cargo.toml (Rust), pom.xml/build.gradle (Java), *.csproj (.NET).

Common Mistakes

#	Mistake	Fix
1	Retrying without idempotency key	Always include idempotency key — prevents duplicate sends on retry. Format: <event-type>/<entity-id>
2	Not verifying webhook signatures	Always verify with resend.webhooks.verify() — unverified events can't be trusted
3	Template variable name mismatch	Variable names are case-sensitive — must match the template definition exactly. Use triple mustache {{{VAR}}} syntax
4	Expecting email body in webhook payload	Webhooks contain metadata only — call resend.emails.receiving.get() for body content
5	Using try/catch for Node.js SDK errors	SDK returns { data, error } — check error explicitly, don't wrap in try/catch
6	Using batch for emails with attachments	Batch doesn't support attachments — use single sends instead
7	Testing with fake emails (test@gmail.com)	Use delivered@resend.dev — fake addresses bounce and hurt reputation
8	Sending with draft template	Templates must be published before sending — call .publish() first
9	html + template in same send call	Mutually exclusive — remove html/text/react when using template
10	MX record not lowest priority for inbound	Ensure Resend's MX has the lowest number (highest priority) or emails won't route

Cross-Cutting Concerns

Send + Receive Together

Auto-replies, email forwarding, or any receive-then-send workflow requires both capabilities:

1. Set up inbound domain first (see receiving.md)
2. Set up sending (see sending/overview.md)
3. Note: batch sending does NOT support attachments or scheduling — use single sends when forwarding with attachments

AI Agent Inbox

If your system processes untrusted email content and takes actions (refunds, database changes, forwarding), install the agent-email-inbox skill. This applies whether or not AI is involved — any system interpreting freeform email content from external senders needs security measures.

Marketing Emails

The sending capabilities in this skill are for transactional email (receipts, confirmations, notifications). For marketing campaigns to large subscriber lists with unsubscribe links and engagement tracking, use Resend Broadcasts.

Domain Warm-up

New domains must gradually increase sending volume. Day 1 limit: ~150 emails (new domain) or ~1,000 (existing domain). See the warm-up schedule in sending/overview.md.

Testing

Never test with fake addresses at real email providers (test@gmail.com, fake@outlook.com) — they bounce and destroy sender reputation.

Address	Result
delivered@resend.dev	Simulates successful delivery
bounced@resend.dev	Simulates hard bounce
complained@resend.dev	Simulates spam complaint

Suppression List

Resend automatically suppresses hard-bounced and spam-complained addresses. Sending to suppressed addresses fires the email.suppressed webhook event instead of attempting delivery. Manage in Dashboard → Suppressions.

Webhook Event Types

Event	Trigger
email.sent	API request successful
email.delivered	Reached recipient's mail server
email.bounced	Permanently rejected (hard bounce)
email.complained	Recipient marked as spam
email.opened / email.clicked	Recipient engagement
email.delivery_delayed	Soft bounce, Resend retries
email.received	Inbound email arrived
domain.* / contact.*	Domain/contact changes

See webhooks.md for full details, signature verification, and retry schedule.

Error Handling Quick Reference

Code	Action
400, 422	Fix request parameters, don't retry
401, 403	Check API key / verify domain, don't retry
409	Idempotency conflict — use new key or fix payload
429	Rate limited — retry with exponential backoff (default rate limit: 2 req/s)
500	Server error — retry with exponential backoff

Resources

• Resend Documentation
• API Reference
• Dashboard