email-infrastructure

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• SKILL.md

SKILL.md (first lines)

# Email Infrastructure

Production email delivery requires DNS authentication, domain isolation, and provider-agnostic architecture. A single misconfiguration can land your entire domain in spam.

## GOOD vs BAD: Domain Strategy

```
BAD:  Send bulk marketing from example.com
      → Spam complaints tank your main domain reputation
      → Transactional emails (password reset, receipts) start landing in spam
      → Recovery takes weeks of warmup

GOOD: Subdomain isolation with separate reputations
      → mail.example.com        for transactional (password reset, receipts, 2FA)
      → notify.example.com      for product notifications (comments, mentions)
      → marketing.example.com   for bulk campaigns (newsletters, promotions)
      → Each subdomain has independent reputation — one bad campaign does not poison the rest
```

## DNS Authentication: SPF + DKIM + DMARC

```
# SPF — declare which servers can send from your domain
mail.example.com     TXT  "v=spf1 include:_spf.provider.com ~all"

# DKIM — cryptographic signature on every email
selector._domainkey.mail.example.com  TXT  "v=DKIM1; k=rsa; p=MIGf..."

# DMARC — policy for failed authentication (progressive rollout)
# Week 1-2: monitor only
_dmarc.mail.example.com  TXT  "v=DMARC1; p=none; rua=mailto:dmarc@example.com"

# Week 3-4: quarantine suspicious emails
_dmarc.mail.example.com  TXT  "v=DMARC1; p=quarantine; pct=25; rua=mailto:dmarc@example.com"

# Week 5+: reject after confidence builds
_dmarc.mail.example.com  TXT  "v=DMARC1; p=reject; rua=mailto:dmarc@example.com"
```

Never jump straight to `p=reject`. The progressive rollout catches misconfigurations before they block legitimate mail.

## Email Provider Abstraction

```typescript
// Swap Resend, SES, Postmark, or Mailgun without touching business logic

interface EmailProvider {
  send(message: EmailMessage): Promise<EmailResult>
  sendBatch(messages: EmailMessage[]): Promise<EmailResult[]>
}

interface EmailMessage {
  from: string
  to: string | string[]
  subject: string
  html: string
  text?: string
  replyTo?: string
  headers?: Record<string, string>
  tags?: Record<string, string>
}

interface EmailResult {
  id: string
  status: 'sent' | 'queued' | 'failed'
  error?: string
}

// Factory selects provider from config — no hardcoded vendor
function createEmailProvider(config: { provider: string }): EmailProvider {
  switch (config.provider) {
    case 'resend':    return new ResendProvider()
    case 'ses':       return new SESProvider()
    case 'postmark':  return new PostmarkProvider()
    default:          throw new Error(`Unknown email provider: ${config.provider}`)
  }
}
```

## Transactional vs Marketing Separation

```typescript
interface EmailService {
  sendTransactional(message: EmailMessage): Promise<EmailResult>
  sendMarketing(message: EmailMessage): Promise<EmailResult>
}

class ProductionEmailService implements EmailService {
  constructor(
    private transactional: EmailProvider,  // high-deliverability provider
    private marketing: EmailProvider       // bulk-optimized provider
  ) {}

  async sendTransactional(message: EmailMessage): Promise<EmailResult> {
    // Transactional: password reset, receipts, 2FA — must arrive instantly
    // Use mail.example.com subdomain, high-priority provider
    return this.transactional.send({
      ...message,
      from: `noreply@mail.example.com`,
      headers: { 'X-Priority': '1' }
    })
  }

  async sendMarketing(message: EmailMessage): Promise<EmailResult> {
    // Marketing: newsletters, promotions — rate-limited, includes unsubscribe
    // Use marketing.example.com subdomain, bulk provider
    return this.marketing.send({
      ...message,
      from: `hello@marketing.example.com`,
      headers: { 'List-Unsubscribe': `<https://example.com/unsubscribe>` }
    })
  }
}
```

## Template System (MJML)

```typescript
// MJML compiles to responsive HTML that works across all email clients

From vibeeval/vibecosystem, MIT.

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