Scaffold complete email infrastructure — Resend setup, transactional templates, user segmentation, and an admin send UI. Reads the project first, plugs into existing auth and database.

Phase 1: Detect the Project

Before writing anything, read the codebase:

1.1 Stack Detection

• Framework: Next.js / Express / FastAPI / other?
• Database: Supabase / Prisma / Drizzle / Mongoose / raw SQL?
• Auth: What fields identify a user? (clerk_user_id, email, id)
• User table schema: What fields exist? (email, name, credits, plan, created_at, last_active_at)

1.2 Ask the User

Before scaffolding, confirm:

I'll wire email for your [framework] app with [database].

Quick decisions:

1. What emails do you need? (transactional, campaigns, or both)
2. Sender: From name and from email? (e.g., "Tushar from Bangers Only <hi@bangersonly.xyz>")
3. Domain verified in Resend? (yes / need to set it up)

Defaults: welcome email + re-engagement campaign, Resend.

Phase 2: Provider Setup

Install and Configure Resend

npm install resend

Add to .env.example:

RESEND_API_KEY=re_xxxxx

Create the email utility — every email in the codebase goes through this:

// lib/email.ts
import { Resend } from 'resend';

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

interface SendEmailParams {
  to: string | string[];
  subject: string;
  html: string;
  from?: string;
  replyTo?: string;
}

export async function sendEmail({
  to,
  subject,
  html,
  from = 'Your Name <you@yourdomain.com>',
  replyTo,
}: SendEmailParams) {
  try {
    const result = await resend.emails.send({ from, to, subject, html, reply_to: replyTo });
    return { success: true, id: result.data?.id };
  } catch (error) {
    console.error('Email send failed:', error);
    return { success: false, error };
  }
}

Never throw from the email utility. Email failures should not crash the main request flow. Return { success, error } and let the caller decide.

Domain Verification Checklist

Tell the user to complete this in the Resend dashboard before going to production:

[ ] Add MX record to DNS
[ ] Add SPF TXT record: v=spf1 include:resend.com ~all
[ ] Add DKIM TXT records (Resend provides these in dashboard)
[ ] Verify domain status shows "Verified" in Resend dashboard
[ ] Set reply-to to a monitored inbox (not noreply)

Without domain verification, emails land in spam or don't send at all.

Phase 3: Email Templates

Create templates for the events that exist in the codebase. At minimum, scaffold these:

Welcome Email (triggers on user signup)

// lib/emails/welcome.ts
export function welcomeEmail({ name, ctaUrl }: { name: string; ctaUrl: string }): string {
  return `
    <div style="font-family: sans-serif; max-width: 560px; margin: 0 auto; padding: 40px 20px; color: #111;">
      <p>Hey ${name},</p>
      <p>You're in. Here's what to do first:</p>
      <p>
        <a href="${ctaUrl}"
           style="display: inline-block; background: #000; color: #fff; padding: 12px 24px;
                  text-decoration: none; border-radius: 6px; font-size: 14px;">
          Get started
        </a>
      </p>
      <p>Any questions — just reply to this email.</p>
      <p style="color: #666; font-size: 13px; margin-top: 40px;">— [Your Name]</p>
      <p style="color: #999; font-size: 11px;">
        <a href="{{{unsubscribe_url}}}" style="color: #999;">Unsubscribe</a>
      </p>
    </div>
  `;
}

Re-engagement Email (triggers for users inactive 7+ days)

// lib/emails/reengagement.ts
export function reengagementEmail({ name, daysSinceActive }: { name: string; daysSinceActive: number }): string {
  return `
    <div style="font-family: sans-serif; max-width: 560px; margin: 0 auto; padding: 40px 20px; color: #111;">
      <p>Hey ${name},</p>
      <p>Haven't seen you in ${daysSinceActive} days. [Product] has [one improvement since they last used it].</p>
      <p>
        <a href="[APP_URL]"
           style="display: inline-block; background: #000; color: #fff; padding: 12px 24px;
                  text-decoration: none; border-radius: 6px; font-size: 14px;">
          Pick up where you left off
        </a>
      </p>
      <p style="color: #666; font-size: 13px; margin-top: 40px;">— [Your Name]</p>
      <p style="color: #999; font-size: 11px;">
        <a href="{{{unsubscribe_url}}}" style="color: #999;">Unsubscribe</a>
      </p>
    </div>
  `;
}

Template Rules

• Plain text feel — no heavy HTML design
• One CTA per email
• Signed by a human name, not "The Team"
• Every template must include an unsubscribe link for campaigns
• Inline styles only — no external stylesheets (spam filters strip them)

Wire Welcome Email Into Signup Flow

Find the signup/auth sync endpoint and call sendEmail after user creation:

// After creating the new user:
if (isNewUser) {
  await sendEmail({
    to: user.email,
    subject: 'Welcome',
    html: welcomeEmail({ name: user.name || 'there', ctaUrl: process.env.APP_URL! }),
  });
}

Phase 4: User Segmentation

Read the user table schema and generate segments based on available fields:

Generate the actual SQL/ORM query for each segment that exists. Example for credits-based app:

-- Dormant users: used some credits but went quiet
SELECT email, name
FROM users
WHERE (initial_credits - credits) > 0
  AND last_active_at < NOW() - INTERVAL '7 days'
  AND email_unsubscribed = false;

Phase 5: Admin Campaign UI

If campaigns are requested, scaffold an admin route:

POST /api/admin/send-campaign
Body: { segment: string, templateId: string }

Handler logic:

1. Check admin auth — never skip this
2. Query users in segment
3. Loop and send — catch per-email errors, don't abort the batch on one failure
4. Return { sentCount, failedEmails, errors }

// api/admin/send-campaign/route.ts
export async function POST(req: Request) {
  const adminSecret = req.headers.get('x-admin-secret');
  if (adminSecret !== process.env.ADMIN_SECRET) {
    return Response.json({ error: 'Unauthorized' }, { status: 401 });
  }

  const { segment, templateId } = await req.json();
  const users = await getUsersInSegment(segment);

  let sentCount = 0;
  const failedEmails: string[] = [];

  for (const user of users) {
    const result = await sendEmail({
      to: user.email,
      subject: getSubjectForTemplate(templateId),
      html: renderTemplate(templateId, user),
    });

    if (result.success) {
      sentCount++;
    } else {
      failedEmails.push(user.email);
    }
  }

  return Response.json({ sentCount, failedEmails });
}

Rate limit awareness: Resend free tier = 100 emails/day. Paid starts at $20/mo for 50,000/day. If segment is larger than the daily limit, add batch delays or warn the user.

Phase 6: Unsubscribe Mechanism

Every campaign email needs a working unsubscribe. This isn't optional — it's both legal and a deliverability requirement.

// lib/unsubscribe.ts
import { createHmac } from 'crypto';

export function generateUnsubToken(email: string): string {
  return createHmac('sha256', process.env.UNSUB_SECRET!)
    .update(email)
    .digest('hex')
    .slice(0, 16);
}

export function unsubUrl(email: string): string {
  const token = generateUnsubToken(email);
  return `${process.env.APP_URL}/unsubscribe?email=${encodeURIComponent(email)}&token=${token}`;
}

Add email_unsubscribed boolean DEFAULT false to the users table if it doesn't exist. The unsubscribe route sets it to true and all segment queries must filter on email_unsubscribed = false.

Phase 7: Verify

Flow 1: Transactional
[ ] User signs up → welcome email arrives in inbox (not spam)
[ ] Email shows from verified domain, not @resend.dev
[ ] Reply-to is a real inbox

Flow 2: Campaigns
[ ] Admin send-campaign endpoint requires auth
[ ] Segment query returns correct users
[ ] Failed individual emails don't abort the batch
[ ] Response shows sentCount and failedEmails

Flow 3: Unsubscribe
[ ] Unsubscribe link in every campaign email
[ ] Clicking it marks user as unsubscribed in DB
[ ] Unsubscribed users excluded from future segments

Flow 4: Edge Cases
[ ] sendEmail never throws — returns { success: false } on failure
[ ] Email sending doesn't block the main signup flow
[ ] RESEND_API_KEY in .env.example (not in code)

Important Notes

• Never hardcode email addresses. Use env vars for from/reply-to.
• Welcome email timing matters. Send it in the background — don't make signup wait for email delivery.
• Test with Resend's test mode before switching to live API key.
• One email per event. Don't fire multiple emails on the same trigger — users notice.

See references/guide.md for email copy by segment, subject line formulas, and rate limiting patterns.