Resend

Capabilities

These values come from the adapter's exported capabilities declaration. The field support matrix covers normalized message fields.

The Resend adapter calls the Resend Email API with plain fetch. It is the default first adapter in these docs: setup is one API key, field support is broad, and it forwards idempotency keys to the provider as the Idempotency-Key header, so Resend itself deduplicates retried sends.

Resend@opencoredev/email-sdk/resend

Provider docs

Configure

Grab an API key from the Resend dashboard and verify the domain you plan to send from.

import { createEmailClient } from "@opencoredev/email-sdk";
import { resend } from "@opencoredev/email-sdk/resend";

export const email = createEmailClient({
  adapters: [resend({ apiKey: process.env.RESEND_API_KEY! })],
});

Send

Resend maps every common transactional field: cc, bcc, replyTo, headers, attachments, and tags. sendAt schedules the send on Resend's side, transmitted as scheduled_at in ISO 8601 form.

const result = await email.send(
  {
    from: "Acme <hello@acme.com>",
    to: [{ email: "user@example.com", name: "Ada" }],
    replyTo: "support@example.com",
    subject: "Welcome to Acme",
    html: "<p>Your workspace is ready.</p>",
    tags: [{ name: "type", value: "welcome" }],
    attachments: [
      { filename: "welcome.txt", content: "Welcome to Acme.", contentType: "text/plain" },
    ],
  },
  { idempotencyKey: "welcome:user_123" },
);

console.log(result.id); // Resend email id

The idempotencyKey is sent as Resend's Idempotency-Key header, so retried requests cannot double-send.

Verify from the CLI

RESEND_API_KEY="re_..." npx email-sdk doctor --adapter resend

RESEND_API_KEY="re_..." npx email-sdk send \
  --adapter resend \
  --from "Acme <hello@acme.com>" \
  --to user@example.com \
  --subject "Resend smoke test" \
  --text "It works" \
  --dry-run

Drop --dry-run to perform one real send from the environment that will send production email. Local validation cannot tell you whether the account, sender domain, and recipient policy are actually ready; one live send can.

Compare Resend with other providers

Side-by-side message-field support from the SDK capability matrix, with working code for both adapters:

• Resend vs Postmark
• Resend vs SendGrid
• Resend vs AWS SES
• Resend vs Mailgun
• Resend vs Loops
• Resend vs MailerSend