Email Systems

Email has the highest ROI of any marketing channel. $36 for every $1 spent. Yet most startups treat it as an afterthought - bulk blasts, no personalization, landing in spam folders.

This skill covers transactional email that works, marketing automation that converts, deliverability that reaches inboxes, and the infrastructure decisions that scale.

Principles

• Transactional vs Marketing separation | Description: Transactional emails (password reset, receipts) need 100% delivery. Marketing emails (newsletters, promos) have lower priority. Use separate IP addresses and providers to protect transactional deliverability. | Examples: Good: Password resets via Postmark, marketing via ConvertKit | Bad: All emails through one SendGrid account
• Permission is everything | Description: Only email people who asked to hear from you. Double opt-in for marketing. Easy unsubscribe. Clean your list ruthlessly. Bad lists destroy deliverability. | Examples: Good: Confirmed subscription + one-click unsubscribe | Bad: Scraped email list, hidden unsubscribe, bought contacts
• Deliverability is infrastructure | Description: SPF, DKIM, DMARC are not optional. Warm up new IPs. Monitor bounce rates. Deliverability is earned through technical setup and good behavior. | Examples: Good: All DNS records configured, dedicated IP warmed for 4 weeks | Bad: Using free tier shared IP, no authentication records
• One email, one goal | Description: Each email should have exactly one purpose and one CTA. Multiple asks means nothing gets clicked. Clear single action. | Examples: Good: "Click here to verify your email" (one button) | Bad: "Verify email, check out our blog, follow us on Twitter, refer a friend..."
• Timing and frequency matter | Description: Wrong time = low open rates. Too frequent = unsubscribes. Let users set preferences. Test send times. Respect inbox fatigue. | Examples: Good: Weekly digest on Tuesday 10am user's timezone, preference center | Bad: Daily emails at random times, no way to reduce frequency

Patterns

Transactional Email Queue

Queue all transactional emails with retry logic and monitoring

When to use: Sending any critical email (password reset, receipts, confirmations)

// Don't block request on email send await queue.add('email', { template: 'password-reset', to: user.email, data: { resetToken, expiresAt } }, { attempts: 3, backoff: { type: 'exponential', delay: 2000 } });

Email Event Tracking

Track delivery, opens, clicks, bounces, and complaints

When to use: Any email campaign or transactional flow

Track lifecycle:

• Queued: Email entered system
• Sent: Handed to provider
• Delivered: Reached inbox
• Opened: Recipient viewed
• Clicked: Recipient engaged
• Bounced: Permanent failure
• Complained: Marked as spam

Template Versioning

Version email templates for rollback and A/B testing

When to use: Changing production email templates

templates/ password-reset/ v1.tsx (current) v2.tsx (testing 10%) v1-deprecated.tsx (archived)

Deploy new version gradually

Monitor metrics before full rollout

Bounce Handling State Machine

Automatically handle bounces to protect sender reputation

When to use: Processing bounce and complaint webhooks

switch (bounceType) { case 'hard': await markEmailInvalid(email); break; case 'soft': await incrementBounceCount(email); if (count >= 3) await markEmailInvalid(email); break; case 'complaint': await unsubscribeImmediately(email); break; }

React Email Components

Build emails with reusable React components

When to use: Creating email templates

import { Button, Html } from '@react-email/components';

export default function WelcomeEmail({ userName }) { return ( Welcome {userName}! Get Started ); }

Preference Center

Let users control email frequency and topics

When to use: Building marketing or notification systems

Preferences: ☑ Product updates (weekly) ☑ New features (monthly) ☐ Marketing promotions ☑ Account notifications (always)

Respect preferences in all sends

Required for GDPR compliance

Sharp Edges

Missing SPF, DKIM, or DMARC records

Severity: CRITICAL

Situation: Sending emails without authentication. Emails going to spam folder. Low open rates. No idea why. Turns out DNS records were never set up.

Symptoms:

• Emails going to spam
• Low deliverability rates
• mail-tester.com score below 8
• No DMARC reports received

Why this breaks: Email authentication (SPF, DKIM, DMARC) tells receiving servers you're legit. Without them, you look like a spammer. Modern email providers increasingly require all three.

Recommended fix:

Required DNS records:

SPF (Sender Policy Framework)

TXT record: v=spf1 include:_spf.google.com include:sendgrid.net ~all

DKIM (DomainKeys Identified Mail)

TXT record provided by your email provider Adds cryptographic signature to emails

DMARC (Domain-based Message Authentication)

TXT record: v=DMARC1; p=quarantine; rua=mailto:dmarc@yourdomain.com

Verify setup:

• Send test email to mail-tester.com
• Check MXToolbox for record validation
• Monitor DMARC reports

Using shared IP for transactional email

Severity: HIGH

Situation: Password resets going to spam. Using free tier of email provider. Some other customer on your shared IP got flagged for spam. Your reputation is ruined by association.

Symptoms:

• Transactional emails in spam
• Inconsistent delivery
• Using same provider for marketing and transactional

Why this breaks: Shared IPs share reputation. One bad actor affects everyone. For critical transactional email, you need your own IP or a provider with strict shared IP policies.

Recommended fix:

Transactional email strategy:

Option 1: Dedicated IP (high volume)

• Get dedicated IP from your provider
• Warm it up slowly (start with 100/day)
• Maintain consistent volume

Option 2: Transactional-only provider

• Postmark (very strict, great reputation)
• Includes shared pool with high standards

Separate concerns:

• Transactional: Postmark or Resend
• Marketing: ConvertKit or Customer.io
• Never mix marketing and transactional

Not processing bounce notifications

Severity: HIGH

Situation: Emailing same dead addresses over and over. Bounce rate climbing. Email provider threatening to suspend account. List is 40% dead.

Symptoms:

• Bounce rate above 2%
• No webhook handlers for bounces
• Same emails failing repeatedly

Why this breaks: Bounces damage sender reputation. Email providers track bounce rates. Above 2% and you start looking like a spammer. Dead addresses must be removed immediately.

Recommended fix:

Bounce handling requirements:

Hard bounces:

Remove immediately on first occurrence Invalid address, domain doesn't exist

Soft bounces:

Retry 3 times over 72 hours After 3 failures, treat as hard bounce

Implementation:

// Webhook handler for bounces
app.post('/webhooks/email', (req, res) => {
  const event = req.body;
  if (event.type === 'bounce') {
    await markEmailInvalid(event.email);
    await removeFromAllLists(event.email);
  }
});

Monitor:

Track bounce rate by campaign Alert if bounce rate exceeds 1%

Missing or hidden unsubscribe link

Severity: CRITICAL

Situation: Users marking as spam because they cannot unsubscribe. Spam complaints rising. CAN-SPAM violation. Email provider suspends account.

Symptoms:

• Hidden unsubscribe links
• Multi-step unsubscribe process
• No List-Unsubscribe header
• High spam complaint rate

Why this breaks: Users who cannot unsubscribe will mark as spam. Spam complaints hurt reputation more than unsubscribes. Also it is literally illegal. CAN-SPAM, GDPR all require clear unsubscribe.

Recommended fix:

Unsubscribe requirements:

Visible:

• Above the fold in email footer
• Clear text, not hidden
• Not styled to be invisible

One-click:

• Link directly unsubscribes
• No login required
• No "are you sure" hoops

List-Unsubscribe header:

List-Unsubscribe: <mailto:unsubscribe@example.com>,
  <https://example.com/unsubscribe?token=xxx>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

Preference center:

Option to reduce frequency instead of full unsubscribe

Sending HTML without plain text alternative

Severity: MEDIUM

Situation: Some users see blank emails. Spam filters flagging emails. Accessibility issues for screen readers. Email clients that strip HTML show nothing.

Symptoms:

• No text/plain part in emails
• Blank emails for some users
• Lower engagement in some segments

Why this breaks: Not everyone can render HTML. Screen readers work better with plain text. Spam filters are suspicious of HTML-only. Multipart is the standard.

Recommended fix:

Always send multipart:

await resend.emails.send({
  from: 'you@example.com',
  to: 'user@example.com',
  subject: 'Welcome!',
  html: '<h1>Welcome!</h1><p>Thanks for signing up.</p>',
  text: 'Welcome!\n\nThanks for signing up.',
});

Auto-generate text from HTML:

Use html-to-text library as fallback But hand-crafted plain text is better

Plain text should be readable:

Not just HTML stripped of tags Actual formatted text content

Sending high volume from new IP immediately

Severity: HIGH

Situation: Just switched providers. Started sending 50,000 emails/day immediately. Massive deliverability issues. New IP has no reputation. Looks like spam.

Symptoms:

• New IP/provider
• Sending high volume immediately
• Sudden deliverability drop

Why this breaks: New IPs have no reputation. Sending high volume immediately looks like a spammer who just spun up. You need to gradually build trust.

Recommended fix:

IP warm-up schedule:

Week 1: 50-100 emails/day Week 2: 200-500 emails/day Week 3: 500-1000 emails/day Week 4: 1000-5000 emails/day Continue doubling until at volume

Best practices:

• Start with most engaged users
• Send to Gmail/Microsoft first (they set reputation)
• Maintain consistent volume
• Don't spike and drop

During warm-up:

• Monitor deliverability closely
• Check feedback loops
• Adjust pace if issues arise

Emailing people who did not opt in

Severity: CRITICAL

Situation: Bought an email list. Scraped emails from LinkedIn. Added conference contacts. Spam complaints through the roof. Provider suspends account. Maybe a lawsuit.

Symptoms:

• Purchased email lists
• Scraped contacts
• High unsubscribe rate on first send
• Spam complaints above 0.1%

Why this breaks: Permission-based email is not optional. It is the law (CAN-SPAM, GDPR). It is also effective - unwilling recipients hurt your metrics and reputation more than they help.

Recommended fix:

Permission requirements:

Explicit opt-in:

• User actively chooses to receive email
• Not pre-checked boxes
• Clear what they are signing up for

Double opt-in:

• Confirmation email with link
• Only add to list after confirmation
• Best practice for marketing lists

What you cannot do:

• Buy email lists
• Scrape emails from websites
• Add conference contacts without consent
• Use partner/customer lists without consent

Transactional exception:

Password resets, receipts, account alerts do not need marketing opt-in

Emails that are mostly or entirely images

Severity: MEDIUM

Situation: Beautiful designed email that is one big image. Users with images blocked see nothing. Spam filters flag it. Mobile loading is slow. No one can copy text.

Symptoms:

• Single image emails
• No text content visible
• Missing or generic alt text
• Low engagement when images blocked

Why this breaks: Images are blocked by default in many clients. Spam filters are suspicious of image-only emails. Accessibility suffers. Load times increase.

Recommended fix:

Balance images and text:

60/40 rule:

• At least 60% text content
• Images for enhancement, not content

Always include:

• Alt text on every image
• Key message in text, not just image
• Fallback for images-off view

Test:

• Preview with images disabled
• Should still be usable

Example:

<img
  src="hero.jpg"
  alt="Save 50% this week - use code SAVE50"
  style="max-width: 100%"
/>
<p>Use code <strong>SAVE50</strong> to save 50% this week.</p>

Missing or default preview text

Severity: MEDIUM

Situation: Inbox shows "View this email in browser" or random HTML as preview. Lower open rates. First impression wasted on boilerplate.

Symptoms:

• View in browser as preview
• HTML code visible in preview
• No preview component in template

Why this breaks: Preview text is prime real estate - appears right after subject line. Default or missing preview text wastes this space. Good preview text increases open rates 10-30%.

Recommended fix:

Add explicit preview text:

In HTML:

<div style="display:none;max-height:0;overflow:hidden;">
  Your preview text here. This appears in inbox preview.
  <!-- Add whitespace to push footer text out -->
  &nbsp;&zwnj;&nbsp;&zwnj;&nbsp;&zwnj;&nbsp;&zwnj;&nbsp;
</div>

With React Email:

<Preview>
  Your preview text here. This appears in inbox preview.
</Preview>

Best practices:

• Complement the subject line
• 40-100 characters optimal
• Create curiosity or value
• Different from first line of email

Not handling partial send failures

Severity: HIGH

Situation: Sending to 10,000 users. API fails at 3,000. No tracking of what sent. Either double-send or lose 7,000. No way to know who got the email.

Symptoms:

• No per-recipient send logging
• Cannot tell who received email
• Double-sending issues
• No retry mechanism

Why this breaks: Bulk sends fail partially. APIs timeout. Rate limits hit. Without tracking individual send status, you cannot recover gracefully.

Recommended fix:

Track each send individually:

async function sendCampaign(emails: string[]) {
  const results = await Promise.allSettled(
    emails.map(async (email) => {
      try {
        const result = await resend.emails.send({ to: email, ... });
        await db.emailLog.create({
          email,
          status: 'sent',
          messageId: result.id,
        });
        return result;
      } catch (error) {
        await db.emailLog.create({
          email,
          status: 'failed',
          error: error.message,
        });
        throw error;
      }
    })
  );

  const failed = results.filter(r => r.status === 'rejected');
  // Retry failed sends or alert
}

Best practices:

• Log every send attempt
• Include message ID for tracking
• Build retry queue for failures
• Monitor success rate per campaign

Validation Checks

Missing plain text email part

Severity: WARNING

Emails should always include a plain text alternative

Message: Email being sent with HTML but no plain text part. Add 'text:' property for accessibility and deliverability.

Hardcoded from email address

Severity: WARNING

From addresses should come from environment variables

Message: From email appears hardcoded. Use environment variable for flexibility.

Missing bounce webhook handler

Severity: WARNING

Email bounces should be handled to maintain list hygiene

Message: Email provider used but no bounce handling detected. Implement webhook handler for bounces.

Missing List-Unsubscribe header

Severity: INFO

Marketing emails should include List-Unsubscribe header

Message: Marketing email detected without List-Unsubscribe header. Add header for better deliverability.

Synchronous email send in request handler

Severity: WARNING

Email sends should be queued, not blocking

Message: Email sent synchronously in request handler. Consider queuing for better reliability.

Email send without retry logic

Severity: INFO

Email sends should have retry mechanism for failures

Message: Email send without apparent retry logic. Add retry for transient failures.

Email API key in code

Severity: ERROR

API keys should come from environment variables

Message: Email API key appears hardcoded in source code. Use environment variable.

Bulk email without rate limiting

Severity: WARNING

Bulk sends should respect provider rate limits

Message: Bulk email sending without apparent rate limiting. Add throttling to avoid hitting limits.

Email without preview text

Severity: INFO

Emails should include preview/preheader text

Message: Email template without preview text. Add hidden preheader for inbox preview.

Email send without logging

Severity: WARNING

Email sends should be logged for debugging and auditing

Message: Email being sent without apparent logging. Log sends for debugging and compliance.

Collaboration

Delegation Triggers

• copy|subject|messaging|content -> copywriting (Email needs copy)
• design|template|visual|layout -> ui-design (Email needs design)
• track|analytics|measure|metrics -> analytics-architecture (Email needs tracking)
• infrastructure|deploy|server|queue -> devops (Email needs infrastructure)

Email Marketing Stack

Skills: email-systems, copywriting, marketing, analytics-architecture

Workflow:

1. Infrastructure setup (email-systems)
2. Template creation (email-systems)
3. Copy writing (copywriting)
4. Campaign launch (marketing)
5. Performance tracking (analytics-architecture)

Transactional Email

Skills: email-systems, backend, devops

Workflow:

1. Provider setup (email-systems)
2. Template coding (email-systems)
3. Queue integration (backend)
4. Monitoring (devops)

When to Use

Use this skill when the request clearly matches the capabilities and patterns described above.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.