@msgly/resend

📖 Docs & channel reference: https://ayushjain070401.github.io/msgly/

Resend adapter for Msgly — transactional email over plain HTTP.

Unlike @msgly/smtp, this one is Edge-compatible: it's fetch and Web Crypto only, no TCP sockets, so it runs on Vercel Edge, Cloudflare Workers, and Deno.

npm install @msgly/core @msgly/resend

import { createHub } from '@msgly/core';
import { createResendAdapter } from '@msgly/resend';

const resend = createResendAdapter({
  apiKey: process.env.RESEND_API_KEY!,
  from: 'Acme <[email protected]>',
  webhookSecret: process.env.RESEND_WEBHOOK_SECRET!,  // whsec_...
  attachments: { enabled: true },
});

const hub = createHub().register(resend);

await hub.send({
  channel: 'resend',
  account: { channel: 'resend', channelAccountId: '[email protected]' },
  contact: { channel: 'resend', channelUserId: '[email protected]' },
  content: { type: 'text', text: '<h1>Welcome</h1>', format: 'html' },
  metadata: { subject: 'Welcome aboard' },
});

Webhooks

Resend signs with Svix. Pass webhookSecret and the adapter verifies the HMAC over "{svix-id}.{svix-timestamp}.{body}", and enforces a timestamp window (default 5 minutes, webhookToleranceSec) so a captured request can't be replayed indefinitely. Multiple signatures are accepted during key rotation.

Two kinds of event arrive on the same endpoint, and they mean different things:

// Inbound mail → unified messages
hub.on('message', (msg) => { /* someone emailed you */ });

// Delivery events → receipts (email.sent / delivered / bounced / opened …)
const receipt = resend.parseDeliveryEvent(req);
if (receipt) console.log(receipt.status, receipt.recipientId);

handleWebhook deliberately only turns email.received into a message; status events are not messages and would otherwise pollute your inbound handler.

Credential checks

verifyCredentials() goes past "is the key valid" and checks that the domain in from is actually registered and verified in the account — the most common reason a first send fails with a confusing 422.

Attachments

Opt in per adapter, like the other email adapters. Resend takes attachment bytes inline as base64, so there's no upload step. Note Resend exposes no attachment download API — for inbound attachments, store the bytes yourself when the webhook arrives.

Rate limits

Resend's default is 2 requests/second, which is the campaign default here. Raise it if Resend has raised yours:

await hub.sendBulk({ channel: 'resend', /* ... */, rateLimit: { perSecond: 10 } });

License

MIT