Resend

Overview

Resend sends email over a REST API with official SDKs (Node.js, Python, Go, Ruby, PHP, Java, .NET, Rust). Templates can be written as React components with React Email, delivery events arrive as signed webhooks, and marketing mail goes out as broadcasts to a segment of contacts. This skill covers the Node.js SDK (resend 6.x) as of October 2026.

Instructions

Install and authenticate

npm install resend react react-dom
npm install react-email            # templates and the preview server

Create an API key in the Resend dashboard and keep it in RESEND_API_KEY. Verify a sending domain at resend.com/domains (SPF and DKIM records) before production; onboarding@resend.dev is for testing only.

Send an email

The SDK does not throw on API errors; it returns { data, error }. Node parameter names are camelCase (scheduledAt, replyTo).

import { Resend } from "resend";
import { WelcomeEmail } from "@/emails/welcome";

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

const { data, error } = await resend.emails.send({
  from: "Northwind <hello@mail.northwind.io>",
  to: "maria.lopez@gmail.com",
  subject: "Welcome to Northwind",
  react: WelcomeEmail({ name: "Maria", loginUrl: "https://app.northwind.io/login" }),
});
if (error) throw new Error(`Resend: ${error.message}`);
console.log(data.id); // e.g. "49a3999c-0ce1-4ea6-ab68-afcd6dc2e794"

Pass the component as a function call (WelcomeEmail({...})), not as JSX. Other content options are html, text, or a stored template ({ id, variables }). Extras: cc, bcc, replyTo, headers, tags (name and value use only letters, numbers, _ and -), attachments (filename plus content or path), and scheduledAt (ISO 8601 or text such as "in 1 hour").

Batch sending

const { data, error } = await resend.batch.send(
  users.map((u) => ({
    from: "Northwind <digest@mail.northwind.io>",
    to: u.email,
    subject: "Your weekly digest",
    react: DigestEmail({ items: u.items }),
  })),
);
// data.data[i].id matches payload[i]

A batch holds up to 100 emails. Attachments and scheduledAt are not supported in batch requests.

React Email templates

// emails/welcome.tsx
import { Html, Head, Body, Container, Heading, Text, Button, Hr } from "react-email";

export function WelcomeEmail({ name, loginUrl }: { name: string; loginUrl: string }) {
  return (
    <Html>
      <Head />
      <Body style={{ fontFamily: "Arial, sans-serif", backgroundColor: "#f4f4f5" }}>
        <Container style={{ maxWidth: 600, padding: 20, backgroundColor: "#ffffff" }}>
          <Heading as="h1">Welcome, {name}</Heading>
          <Text>Your workspace is ready.</Text>
          <Button href={loginUrl} style={{ backgroundColor: "#000", color: "#fff", padding: "12px 24px" }}>
            Open Northwind
          </Button>
          <Hr />
          <Text style={{ fontSize: 12, color: "#666" }}>Did not sign up? Ignore this email.</Text>
        </Container>
      </Body>
    </Html>
  );
}

Preview in the browser with npx email dev (serves templates from emails/ on localhost:3000). The older @react-email/components package is deprecated on npm; import components from react-email.

Webhooks

Add an endpoint in the dashboard and store its signing secret as RESEND_WEBHOOK_SECRET. Verify against the raw body, not parsed JSON:

export async function POST(req: Request) {
  const payload = await req.text();
  try {
    const event = resend.webhooks.verify({
      payload,
      headers: {
        id: req.headers.get("svix-id")!,
        timestamp: req.headers.get("svix-timestamp")!,
        signature: req.headers.get("svix-signature")!,
      },
      webhookSecret: process.env.RESEND_WEBHOOK_SECRET!,
    });
    if (event.type === "email.bounced") await markUndeliverable(event.data.to);
  } catch {
    return new Response("Invalid webhook", { status: 400 });
  }
  return new Response("ok");
}

Email events: email.sent, email.scheduled, email.delivered, email.delivery_delayed, email.bounced, email.complained, email.suppressed, email.failed, email.opened, email.clicked, email.received.

Contacts and broadcasts

Contacts are global to the account and are grouped into segments (the older "audiences" are replaced by segments and topics).

await resend.contacts.create({ email: "steve.wozniak@gmail.com", firstName: "Steve", unsubscribed: false });

await resend.broadcasts.create({
  segmentId: "78261eea-8f8b-4381-83c6-79fa7120f1cf",
  from: "Northwind <news@mail.northwind.io>",
  subject: "October release notes",
  html: "Hi {{{contact.first_name|there}}}, read the notes. Unsubscribe: {{{RESEND_UNSUBSCRIBE_URL}}}",
  send: true,
});

Leave out send: true to save a draft.

Examples

Example 1: Send an invoice with a PDF and tags

Request: "Email the customer their invoice PDF and tag it as billing so I can filter in Resend."

import { readFile } from "node:fs/promises";

const { data, error } = await resend.emails.send({
  from: "Northwind Billing <billing@mail.northwind.io>",
  to: ["maria.lopez@gmail.com"],
  bcc: "archive@northwind.io",
  subject: "Invoice INV-2026-0142",
  html: "<h1>Invoice INV-2026-0142</h1><p>Amount due: $99.99</p>",
  attachments: [{ filename: "INV-2026-0142.pdf", content: await readFile("./out/INV-2026-0142.pdf") }],
  tags: [{ name: "category", value: "billing" }],
});

Result: data.id holds the email ID; the message shows up in the Logs page with the category:billing tag.

Example 2: Stop mailing hard bounces

Request: "When an address bounces, flag the user so we stop sending."

Add the webhook route from the Webhooks section, subscribe it to email.bounced and email.complained, and in the handler set the user's emailValid field to false. Test it locally by exposing the dev server with a tunnel and sending to bounced@resend.dev.

Guidelines

• Default rate limit is 10 requests per second per team; a 429 means back off using the retry-after header. Use batch sending for volume.
• A single email accepts at most 50 recipients and 40 MB of attachments after Base64 encoding.
• The Idempotency-Key header (unique per request, kept for 24 hours) prevents duplicate sends on retries.
• Always check error; a failed call resolves normally.
• Verify webhook signatures on the raw request body; any re-serialisation breaks them.
• Send marketing mail through broadcasts so the unsubscribe link and suppression handling are applied.
• Keep the API key server-side only; never ship it to the browser.