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
Get, list, reschedule, cancel emails, retrieve metrics	sending/email-management.md
Receive inbound emails	receiving.md — domain setup, webhooks, attachments
Manage templates (CRUD, variables)	templates.md — lifecycle, aliases, pagination
Set up webhooks (events, verification)	webhooks.md — verification, CRUD, retry schedule, IP allowlist
Manage domains (create, verify, claim, DNS)	domains.md — regions, TLS, tracking, claiming, capabilities
Manage contacts (CRUD, properties)	contacts.md — segments, topics, custom properties, bulk CSV import
Send broadcasts (marketing campaigns)	broadcasts.md — lifecycle, scheduling, template variables
Manage API keys	api-keys.md — permission scoping, domain restrictions
View API request logs	logs.md — list and retrieve API call history, debugging
Define contact properties	contact-properties.md — custom fields for contacts
Manage segments (contact groups)	segments.md — broadcast targeting, contact grouping
Manage topics (subscriptions)	topics.md — opt-in/out preferences, broadcast filtering
Create automations (event-driven workflows)	automations.md — steps, connections, runs, conditions
Define and send events (automation triggers)	events.md — schemas, payloads, contact association
Check account usage and quotas	usage.md — emails, contacts, segments, broadcasts, AI credits, automation runs, domains, rate limit
Install SDK (8+ languages)	installation.md
Set up an AI agent inbox	Install the agent-email-inbox skill — covers security levels for untrusted input

SDK Version Requirements

Always install the latest SDK version. Older SDKs may be missing webhooks.verify(), emails.receiving.get(), or domains.claims.*.

If the project already has a Resend SDK installed, check its version against the minimum versions in installation.md and upgrade if it's below.

See installation.md for the minimum version per language, 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
11	403 when sending from resend.dev	The default onboarding@resend.dev is a sandbox — it can only deliver to your Resend account email. Verify your own domain first
12	403 domain mismatch	The from address domain must exactly match a verified domain. Verified send.acme.com but sending from user@acme.com will fail
13	Calling Resend API from the browser (CORS)	The API does not support CORS — this is intentional to protect your API key. Always call from server-side (API routes, serverless functions)
14	401 restricted_api_key	A sending-only API key was used on a non-sending endpoint (domains, contacts, etc.). Create a full-access key instead

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 — see broadcasts.md for the API.

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	Check API key — restricted_api_key means sending-only key used on non-sending endpoint
403	Verify domain ownership — common causes: resend.dev sandbox, from domain mismatch, unverified domain
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