Resend

Resend — transactional email

Get an app sending email from its own domain, end to end, and prove it with a real delivery.

Argument: $ARGUMENTS is the project path and optionally the sending domain (e.g. ~/projects/my-app mytool.app). Default to the current directory. Ask for the domain only if nothing in the project (.env*, package.json homepage, existing og:url) reveals it.

Resend only sends. If the user also wants hello@domain to receive replies, run /improvmx afterwards and merge the SPF records (see Gotchas).

Process

1. Detect state

cat package.json | grep -E '"(next|resend|react-email|@react-email/components)"'
grep -l RESEND_API_KEY .env .env.local .env.example 2>/dev/null
ls src/app/api 2>/dev/null; ls pages/api 2>/dev/null

Decide the target from what you find:

• Next.js App Router → src/app/api/send/route.ts (or app/api/send/route.ts if there is no src/)
• Next.js Pages Router → pages/api/send.ts
• Anything else (Vite, Astro, Express, plain Node) → lib/email.ts helper plus a note that the send must happen server-side; the API key can never ship to the browser

2. Install and configure

npm install resend

Add to .env.local (create it if missing) and to .env.example with the value blanked:

RESEND_API_KEY=re_xxxxxxxx
EMAIL_FROM="Your Name <hello@yourdomain.com>"

Make sure .env.local is in .gitignore. If it isn't, add it before anything else.

3. Scaffold the send code

App Router route (src/app/api/send/route.ts):

import { Resend } from 'resend';

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

export async function POST(req: Request) {
  const { to, subject, html, replyTo } = await req.json();
  if (!to || !subject || !html) {
    return Response.json({ error: 'to, subject and html are required' }, { status: 400 });
  }
  const { data, error } = await resend.emails.send({
    from: process.env.EMAIL_FROM!,
    to,
    subject,
    html,
    replyTo,
  });
  if (error) return Response.json({ error }, { status: 500 });
  return Response.json({ id: data?.id });
}

Generic helper (lib/email.ts) for non-Next projects: same resend.emails.send call wrapped in an exported sendEmail({ to, subject, html }) function.

If the user described a specific email (contact form, welcome, receipt), write that concrete route instead of the generic one, and add a react-email template under emails/ when the body is more than a paragraph. Hand-written HTML email breaks in Outlook; @react-email/components renders inline-styled HTML that survives every client.

4. Domain verification (the human part)

Tell the user exactly what to do, in order. You cannot do the dashboard steps for them:

1. Sign up at resend.com, then Domains → Add Domain and enter the sending domain.
2. Resend shows three DNS records: an MX for bounces, a TXT for SPF, and a TXT for DKIM. Copy each into the registrar's DNS panel (run /domain if they don't know where that is).
3. On Cloudflare, every one of these must be DNS only (grey cloud). Proxied records break email.
4. Wait about five minutes, click Verify.
5. API Keys → Create API Key, sending-only permission, paste it into .env.local.

Until the domain is verified, the user can still test: Resend lets you send from onboarding@resend.dev to your own signup email only.

5. Verify with a real send

Start the dev server if it isn't running, then:

curl -s -X POST http://localhost:3000/api/send \
  -H 'content-type: application/json' \
  -d '{"to":"delivered@resend.dev","subject":"Resend wired up","html":"<p>It works.</p>"}'

delivered@resend.dev is Resend's sink address: it shows in the dashboard under Emails without delivering anywhere. A returned id means the whole chain works. Then send one to the user's real inbox so they can mark it "not spam" once; Gmail learns from that and later mail lands in the inbox.

6. Report

Resend
- SDK: installed (resend@x.y.z)
- Route: src/app/api/send/route.ts
- Env: RESEND_API_KEY set locally · add to Vercel → Settings → Environment Variables before deploying
- Domain: mytool.app — verified / pending DNS
- Test send: id re_abc123 → dashboard
- Next: /improvmx for inbound, or set Resend as SMTP in Supabase Auth if using magic links

Gotchas

• Only one SPF record per domain. If ImprovMX (or Google Workspace) is also on the domain, merge into a single TXT: v=spf1 include:spf.improvmx.com include:_spf.resend.com ~all. Two v=spf1 records break both services.
• DMARC. Resend nags for it. For a small project v=DMARC1; p=none; rua=mailto:you@yourdomain.com as a TXT at _dmarc is enough.
• from must be on the verified domain. Any local part works (hello@, no-reply@); no mailbox needs to exist.
• Free tier: 3,000 emails/month, 100/day. Pro is $20/month for 50,000. Marketing newsletters belong in a newsletter tool, not here.
• Vercel: the env var has to be added in the Vercel dashboard too. A working local send and a failing production send is almost always that.