---
id: backend-transactional-email
title: Transactional Email — Resend / SendGrid / 템플릿
category: Coding
status: draft
source_trust_level: B
verification_status: conceptual
created_at: 2026-05-09
updated_at: 2026-05-09
tags: [backend, email, transactional, vibe-coding]
tech_stack: { language: "TS / React Email", applicable_to: ["Backend"] }
applied_in: []
aliases: [transactional email, Resend, SendGrid, Postmark, React Email, MJML, SPF DKIM]
---

# Transactional Email

> 사용자 행동 트리거 (가입, 영수증). **마케팅과 분리, 별 IP / 별 도메인 권장**. 도구: Resend / Postmark / SendGrid. 템플릿: React Email / MJML. SPF/DKIM/DMARC 필수.

## 📖 핵심 개념
- Transactional vs Marketing: 다른 IP / 도메인 / sender reputation.
- SPF / DKIM / DMARC: spoofing 방어 + spam 회피.
- Bounce / Complaint: 처리 안 하면 reputation 망가짐.
- Idempotency: 같은 이벤트로 두 번 보내면 안 됨.

## 💻 코드 패턴

### Resend (modern)
```ts
import { Resend } from 'resend';

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

await resend.emails.send({
  from: 'Acme <noreply@mail.acme.com>',
  to: user.email,
  subject: 'Welcome to Acme',
  html: render(<WelcomeEmail name={user.name} />),
  headers: { 'X-Idempotency-Key': eventId },
});
```

### React Email 템플릿
```tsx
import { Body, Container, Head, Heading, Html, Link, Preview, Text } from '@react-email/components';

export function WelcomeEmail({ name }: { name: string }) {
  return (
    <Html>
      <Head />
      <Preview>Welcome to Acme, {name}</Preview>
      <Body style={{ backgroundColor: '#f6f9fc', fontFamily: 'system-ui' }}>
        <Container style={{ padding: '40px 20px' }}>
          <Heading>Welcome, {name}!</Heading>
          <Text>Click <Link href="https://acme.com/start">here</Link> to start.</Text>
        </Container>
      </Body>
    </Html>
  );
}
```

```ts
import { render } from '@react-email/render';
const html = await render(<WelcomeEmail name="A" />);
const text = await render(<WelcomeEmail name="A" />, { plainText: true });
```

### 큐 + retry
```ts
// 직접 보내지 말고 큐
queue.add('email', { to, template, data, idempotencyKey });

queue.process('email', async (job) => {
  const sent = await db.emails.find(job.data.idempotencyKey);
  if (sent) return; // 이미 보냄

  const r = await resend.emails.send({...});
  await db.emails.insert({ idempotencyKey: job.data.idempotencyKey, providerId: r.id });
});
```

### Bounce / complaint webhook
```ts
app.post('/webhooks/email', async (req, res) => {
  // SES SNS / SendGrid event webhook / Resend webhook
  const ev = req.body;
  if (ev.type === 'email.bounced') {
    await db.users.update(ev.email, { emailValid: false });
  }
  if (ev.type === 'email.complained') {
    await db.users.update(ev.email, { emailMarketingOptOut: true, emailValid: false });
  }
  res.json({ ok: true });
});
```

### SPF / DKIM / DMARC DNS
```
# SPF
mail.acme.com.  TXT  "v=spf1 include:_spf.resend.com -all"

# DKIM (Resend / SendGrid 가 제공하는 CNAME)
resend._domainkey.mail.acme.com.  CNAME  resend._domainkey.acme.com.resend.email.

# DMARC
_dmarc.mail.acme.com.  TXT  "v=DMARC1; p=quarantine; rua=mailto:dmarc@acme.com"
```

### Marketing 분리
```
Transactional: noreply@mail.acme.com
Marketing:     hello@news.acme.com
```

다른 sub-domain → bounce 가 transactional 평판에 안 영향.

### Plain text + HTML 둘 다
```ts
await resend.emails.send({
  from, to, subject,
  html: render(<Email />),
  text: render(<Email />, { plainText: true }), // spam 점수 낮춤
});
```

### Preview tools
- React Email dev server: 라이브 preview.
- Mailtrap / Litmus: 다양한 client 렌더 테스트.

### 다국어
```ts
const html = await render(<WelcomeEmail name={name} t={i18n.t} />, { locale: user.locale });
```

## 🤔 의사결정 기준
| 종류 | 추천 |
|---|---|
| 작은 SaaS / modern stack | Resend |
| 큰 규모 / 분석 강함 | SendGrid / Mailgun |
| Bounce 까다로움 | Postmark (transactional 전문) |
| 자체 메일 서버 | SES (싸지만 복잡) |
| Email 아닌 SMS | Twilio / MessageBird |
| 마케팅 자동화 | Customer.io / Loops |

## ❌ 안티패턴
- **DKIM 없음**: spam 폴더 직행.
- **Marketing 같은 도메인**: bounce 가 transactional 평판 망가짐.
- **Bounce 처리 안 함**: 평판 추락 → 모두 spam.
- **Throttle 없이 대량**: rate limit hit.
- **Idempotency 없음**: 같은 이벤트 여러 번 메일.
- **Plain text 누락**: 일부 client / spam 점수 ↑.
- **Inline image base64**: 큰 메일 + 일부 client 차단. 호스팅된 URL.
- **Subject 가 spam-trigger**: "FREE!!!" 같은.

## 🤖 LLM 활용 힌트
- Resend + React Email + 큐 + bounce webhook 4종.
- SPF/DKIM/DMARC 자동 사이트 (mxtoolbox) 검증.
- transactional 따로 sub-domain.

## 🔗 관련 문서
- [[Backend_Webhook_Patterns]]
- [[Backend_Job_Queue_Patterns]]
- [[Frontend_i18n_Patterns]]
