Email Systems

Overview

This skill covers building reliable email systems for web applications, including transactional email sending, template design, deliverability optimization, and inbox placement. It addresses integration with email service providers (Resend, AWS SES, SendGrid, Postmark), building responsive HTML email templates with React Email and MJML, configuring DNS records for deliverability (SPF, DKIM, DMARC), managing email types (transactional, notification, marketing), queue management, and bounce/complaint handling.

Use this skill when sending welcome emails, password resets, order confirmations, notification digests, marketing campaigns, or any system-generated email. Also use when debugging deliverability issues or migrating between email providers.

Core Principles

Transactional and marketing are different - Transactional emails (password reset, receipt) must arrive instantly and reliably. Marketing emails (newsletter, promo) can be batched and should have unsubscribe links. Never mix them on the same sending domain.
HTML email is not web development - Email clients render HTML from 2004. No flexbox, no grid, no modern CSS. Use tables for layout, inline styles, and test across clients. React Email and MJML abstract this pain.
Deliverability is earned - SPF, DKIM, and DMARC are table stakes, not guarantees. Maintain low bounce rates (< 2%), low complaint rates (< 0.1%), and warm up new sending domains gradually.
Queue everything - Never send email synchronously in a request handler. Queue email sends to avoid blocking user requests and to enable retry on failure.
Test before sending - Use Mailtrap or Ethereal in development, preview in multiple clients (Litmus, Email on Acid), and verify every link works.

Key Patterns

Pattern 1: React Email Templates with Resend

When to use: Building type-safe, component-based email templates with modern DX and reliable delivery.

Implementation:

// emails/welcome.tsx - React Email template
import {
  Html,
  Head,
  Body,
  Container,
  Section,
  Text,
  Button,
  Img,
  Hr,
  Link,
  Preview,
} from "@react-email/components";

interface WelcomeEmailProps {
  userName: string;
  loginUrl: string;
  guideUrl: string;
}

export function WelcomeEmail({ userName, loginUrl, guideUrl }: WelcomeEmailProps) {
  return (
    <Html lang="en">
      <Head />
      <Preview>Welcome to MyApp, {userName}! Here&apos;s how to get started.</Preview>
      <Body style={styles.body}>
        <Container style={styles.container}>
          <Img
            src="https://myapp.com/logo.png"
            width={120}
            height={36}
            alt="MyApp"
          />

          <Section style={styles.section}>
            <Text style={styles.heading}>Welcome, {userName}!</Text>
            <Text style={styles.text}>
              Thanks for signing up. Your account is ready to go.
              Here are three things to get you started:
            </Text>

            <Text style={styles.listItem}>
              <strong>1. Create your first project</strong> - Click "New Project" on your dashboard.
            </Text>
            <Text style={styles.listItem}>
              <strong>2. Invite your team</strong> - Share your workspace with colleagues.
            </Text>
            <Text style={styles.listItem}>
              <strong>3. Explore the guide</strong> - Learn tips and best practices.
            </Text>

            <Section style={styles.buttonContainer}>
              <Button style={styles.button} href={loginUrl}>
                Go to Dashboard
              </Button>
            </Section>
          </Section>

          <Hr style={styles.hr} />

          <Section style={styles.footer}>
            <Text style={styles.footerText}>
              Need help? Reply to this email or visit our{" "}
              <Link href={guideUrl} style={styles.link}>help center</Link>.
            </Text>
            <Text style={styles.footerText}>
              MyApp, Inc. | 123 Street, City, ST 12345
            </Text>
          </Section>
        </Container>
      </Body>
    </Html>
  );
}

const styles = {
  body: {
    backgroundColor: "#f6f9fc",
    fontFamily: '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif',
  },
  container: {
    backgroundColor: "#ffffff",
    margin: "0 auto",
    padding: "20px 0 48px",
    maxWidth: "600px",
    borderRadius: "8px",
  },
  section: { padding: "0 48px" },
  heading: { fontSize: "24px", fontWeight: "bold" as const, marginBottom: "16px" },
  text: { fontSize: "16px", lineHeight: "26px", color: "#404040" },
  listItem: { fontSize: "15px", lineHeight: "24px", color: "#404040", marginBottom: "8px" },
  buttonContainer: { textAlign: "center" as const, marginTop: "24px", marginBottom: "24px" },
  button: {
    backgroundColor: "#0066ff",
    borderRadius: "6px",
    color: "#ffffff",
    fontSize: "16px",
    fontWeight: "bold" as const,
    textDecoration: "none",
    textAlign: "center" as const,
    display: "inline-block",
    padding: "12px 24px",
  },
  hr: { borderColor: "#e6ebf1", margin: "32px 0" },
  footer: { padding: "0 48px" },
  footerText: { fontSize: "12px", color: "#8898aa", lineHeight: "20px" },
  link: { color: "#0066ff" },
};

// Default props for preview
WelcomeEmail.PreviewProps = {
  userName: "Jane",
  loginUrl: "https://myapp.com/dashboard",
  guideUrl: "https://myapp.com/guide",
} satisfies WelcomeEmailProps;

export default WelcomeEmail;

// lib/email.ts - Email sending service
import { Resend } from "resend";
import { WelcomeEmail } from "@/emails/welcome";
import { PasswordResetEmail } from "@/emails/password-reset";

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

// Type-safe email sending
type EmailTemplate =
  | { template: "welcome"; props: { userName: string; loginUrl: string; guideUrl: string } }
  | { template: "password-reset"; props: { resetUrl: string; expiresIn: string } }
  | { template: "invoice"; props: { invoiceUrl: string; amount: string; dueDate: string } };

const templates: Record<string, React.FC<Record<string, unknown>>> = {
  welcome: WelcomeEmail,
  "password-reset": PasswordResetEmail,
};

const subjects: Record<string, (props: Record<string, unknown>) => string> = {
  welcome: () => "Welcome to MyApp!",
  "password-reset": () => "Reset your password",
  invoice: (p) => `Invoice for $${p.amount}`,
};

async function sendEmail(
  to: string,
  email: EmailTemplate
): Promise<{ id: string }> {
  const Template = templates[email.template];
  const subject = subjects[email.template](email.props);

  const { data, error } = await resend.emails.send({
    from: "MyApp <hello@myapp.com>",
    to,
    subject,
    react: Template(email.props),
  });

  if (error) {
    throw new EmailSendError(error.message, { to, template: email.template });
  }

  return { id: data!.id };
}

export { sendEmail };

Why: React Email provides component-based email templates with TypeScript safety, hot reloading during development, and automatic conversion to email-safe HTML. Resend provides high deliverability, simple API, and React Email integration out of the box. Type-safe template definitions prevent sending emails with missing properties.

Pattern 2: Email Queue with Retry Logic

When to use: Any production email sending. Never send email synchronously in a request handler.

Implementation:

// jobs/email.ts - Background email processing
import { Queue, Worker } from "bullmq";
import { Redis } from "ioredis";
import { sendEmail } from "@/lib/email";

const connection = new Redis(process.env.REDIS_URL!, { maxRetriesPerRequest: null });

// Define the email queue
const emailQueue = new Queue("email", {
  connection,
  defaultJobOptions: {
    attempts: 3,
    backoff: {
      type: "exponential",
      delay: 60_000, // 1 min, then 2 min, then 4 min
    },
    removeOnComplete: { age: 7 * 24 * 60 * 60 }, // Keep completed jobs for 7 days
    removeOnFail: { age: 30 * 24 * 60 * 60 }, // Keep failed jobs for 30 days
  },
});

// Queue an email (call this from your application code)
async function queueEmail(
  to: string,
  template: EmailTemplate,
  options?: { delay?: number; priority?: number }
): Promise<string> {
  const job = await emailQueue.add(
    template.template,
    { to, template },
    {
      delay: options?.delay,
      priority: options?.priority ?? 0,
    }
  );
  return job.id!;
}

// Process emails in the background
const emailWorker = new Worker(
  "email",
  async (job) => {
    const { to, template } = job.data;

    try {
      const result = await sendEmail(to, template);
      return result;
    } catch (error) {
      // Log for monitoring
      console.error(`Email send failed (attempt ${job.attemptsMade + 1}/${job.opts.attempts}):`, {
        to,
        template: template.template,
        error: (error as Error).message,
      });
      throw error; // BullMQ will retry based on backoff config
    }
  },
  {
    connection,
    concurrency: 10, // Process 10 emails concurrently
    limiter: {
      max: 100, // Max 100 emails per minute (respect ESP rate limits)
      duration: 60_000,
    },
  }
);

// Listen for permanent failures
emailWorker.on("failed", (job, error) => {
  if (job && job.attemptsMade >= (job.opts.attempts ?? 3)) {
    // All retries exhausted - alert and log
    alertOps(`Email permanently failed: ${job.data.to} - ${error.message}`);
  }
});

export { queueEmail };

// Usage in application code
async function handleUserSignup(user: User) {
  // Create user in database (synchronous, in request)
  await db.user.create({ data: user });

  // Queue welcome email (async, background)
  await queueEmail(user.email, {
    template: "welcome",
    props: {
      userName: user.name,
      loginUrl: `${process.env.APP_URL}/dashboard`,
      guideUrl: `${process.env.APP_URL}/guide`,
    },
  });
}

Why: Queuing emails decouples delivery from user requests. If the email provider is slow or down, user signup still succeeds. Exponential backoff handles transient failures. Rate limiting prevents exceeding ESP quotas. Job retention enables debugging delivery issues.

Pattern 3: DNS Configuration for Deliverability

When to use: Setting up a new sending domain or diagnosing deliverability issues.

Implementation:

# SPF Record - Authorize sending servers
# Add to DNS as TXT record on your domain
myapp.com.  TXT  "v=spf1 include:_spf.resend.com include:amazonses.com ~all"

# DKIM Record - Cryptographic email signing
# Provider-specific, usually a CNAME record
resend._domainkey.myapp.com.  CNAME  resend._domainkey.example.com.

# DMARC Record - Alignment policy
# Start with p=none (monitor), move to p=quarantine, then p=reject
_dmarc.myapp.com.  TXT  "v=DMARC1; p=quarantine; rua=mailto:dmarc@myapp.com; pct=100; adkim=s; aspf=s"

# Return-Path / Bounce domain (custom envelope sender)
bounce.myapp.com.  MX  10  feedback-smtp.us-east-1.amazonses.com.
bounce.myapp.com.  TXT  "v=spf1 include:amazonses.com ~all"

// Verify DNS configuration programmatically
import { resolveTxt, resolveCname, resolveMx } from "dns/promises";

async function verifyEmailDNS(domain: string): Promise<Record<string, boolean>> {
  const results: Record<string, boolean> = {};

  // Check SPF
  try {
    const txt = await resolveTxt(domain);
    const spf = txt.flat().find((r) => r.startsWith("v=spf1"));
    results.spf = !!spf;
  } catch {
    results.spf = false;
  }

  // Check DMARC
  try {
    const txt = await resolveTxt(`_dmarc.${domain}`);
    const dmarc = txt.flat().find((r) => r.startsWith("v=DMARC1"));
    results.dmarc = !!dmarc;
  } catch {
    results.dmarc = false;
  }

  // Check DKIM (provider-specific selector)
  try {
    const cname = await resolveCname(`resend._domainkey.${domain}`);
    results.dkim = cname.length > 0;
  } catch {
    results.dkim = false;
  }

  return results;
}

Why: Email authentication (SPF, DKIM, DMARC) proves to receiving mail servers that your emails are legitimate. Without them, emails land in spam. SPF says which servers can send for your domain. DKIM cryptographically signs messages. DMARC tells receivers what to do with unauthenticated email.

Pattern 4: Bounce and Complaint Handling

When to use: Maintaining sender reputation and deliverability over time.

Implementation:

// Webhook handler for bounces and complaints (SES/Resend/SendGrid)
async function handleEmailEvent(event: EmailEvent): Promise<void> {
  switch (event.type) {
    case "bounce": {
      const { email, bounceType } = event;

      if (bounceType === "permanent") {
        // Hard bounce - never send to this address again
        await db.emailSuppression.upsert({
          where: { email },
          create: { email, reason: "hard_bounce", suppressedAt: new Date() },
          update: { reason: "hard_bounce", suppressedAt: new Date() },
        });

        // Update user record
        await db.user.updateMany({
          where: { email },
          data: { emailVerified: false, emailBounced: true },
        });
      } else {
        // Soft bounce - retry later, suppress after 3 soft bounces
        const count = await db.emailEvent.count({
          where: { email, type: "soft_bounce", createdAt: { gte: thirtyDaysAgo() } },
        });

        if (count >= 3) {
          await db.emailSuppression.create({
            data: { email, reason: "repeated_soft_bounce", suppressedAt: new Date() },
          });
        }
      }
      break;
    }

    case "complaint": {
      // Spam complaint - suppress immediately
      await db.emailSuppression.upsert({
        where: { email: event.email },
        create: { email: event.email, reason: "complaint", suppressedAt: new Date() },
        update: { reason: "complaint", suppressedAt: new Date() },
      });

      // Remove from all marketing lists
      await db.marketingSubscription.updateMany({
        where: { email: event.email },
        data: { unsubscribedAt: new Date(), reason: "spam_complaint" },
      });
      break;
    }
  }
}

// Check suppression list before sending
async function canSendTo(email: string): Promise<boolean> {
  const suppressed = await db.emailSuppression.findUnique({
    where: { email },
  });
  return !suppressed;
}

Why: ESPs monitor bounce and complaint rates. Exceeding thresholds (bounce > 5%, complaints > 0.1%) triggers sending suspensions. A suppression list prevents sending to known-bad addresses, protecting your sender reputation and ensuring legitimate emails reach inboxes.

Email Provider Comparison

Provider	Best For	Pricing	React Email	Key Feature
Resend	Developer-friendly transactional	100/day free, $20/mo for 50k	Native	Best DX, React Email team
AWS SES	High volume, cost optimization	$0.10 per 1,000	Via render	Cheapest at scale
SendGrid	Marketing + transactional	100/day free	Via render	Marketing automation
Postmark	Transactional reliability	$15/mo for 10k	Via render	Best transactional deliverability

Anti-Patterns

Anti-Pattern	Why It's Bad	Better Approach
Sending email in the request handler	Blocks user response, no retry	Queue with background worker
Same domain for transactional + marketing	Marketing reputation affects password resets	Separate subdomains (mail.app.com vs news.app.com)
No suppression list	Repeated bounces destroy sender reputation	Maintain and check suppression list before every send
Using `<div>` layout in email HTML	Broken rendering in Outlook, Gmail	Use `<table>` layout or React Email/MJML
No unsubscribe link in notification emails	CAN-SPAM violation, spam complaints	One-click unsubscribe header + visible link
Testing with real email addresses	Spam to real people, complaints	Use Mailtrap, Ethereal, or Resend test mode
Sending from a no-reply address	Users can't respond, feels impersonal	Use a monitored reply-to address

Checklist

Related Resources

Skills: authentication-patterns (password reset emails), payment-integration (invoice/receipt emails)
Skills: event-driven-architecture (email as async event)
Rules: docs/reference/stacks/react-typescript.md (React Email component patterns)

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Email Systems

Overview

Core Principles

Key Patterns

Pattern 1: React Email Templates with Resend

Pattern 2: Email Queue with Retry Logic

Pattern 3: DNS Configuration for Deliverability

Pattern 4: Bounce and Complaint Handling

Email Provider Comparison

Anti-Patterns

Checklist

Related Resources

Source Transparency

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