Email Delivery Best Practices for Developers

Your email infrastructure is only as good as your deliverability rate. If emails don't reach the inbox, nothing else matters.

This guide covers the technical best practices for maximizing email delivery rates.

Understanding Email Deliverability

Email deliverability is the ability to deliver emails to recipients' inboxes. It's affected by:

1. Sender reputation — Your IP and domain history
2. Authentication — SPF, DKIM, DMARC
3. Content quality — Spam triggers, formatting
4. Recipient engagement — Opens, clicks, complaints

Let's break down each factor.

Email Authentication: The Foundation

SPF (Sender Policy Framework)

SPF tells receiving servers which IP addresses are authorized to send email for your domain.

example.com. IN TXT "v=spf1 include:_spf.simpleemailapi.dev ~all"

Best practices:

• Keep your SPF record under 10 DNS lookups
• Use ~all (softfail) during testing, -all (hardfail) in production
• Include all legitimate sending sources

DKIM (DomainKeys Identified Mail)

DKIM adds a cryptographic signature to prove emails weren't modified in transit.

s1._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGf..."

Best practices:

• Use 2048-bit keys (1024-bit is deprecated)
• Rotate keys annually
• Sign all outgoing emails

DMARC (Domain-based Message Authentication)

DMARC tells receivers what to do when SPF or DKIM fails.

_dmarc.example.com. IN TXT "v=DMARC1; p=reject; rua=mailto:dmarc@example.com"

Best practices:

• Start with p=none to monitor before enforcing
• Set up aggregate reports (rua) to monitor issues
• Move to p=reject once stable

Sender Reputation Management

IP Warming

New IP addresses need to build reputation gradually. If you suddenly send 100,000 emails from a new IP, you'll get blocked.

Warming schedule example:

When using SimpleEmailAPI, we handle IP warming automatically with our shared IP pool. For high-volume senders, we offer dedicated IPs with guided warming.

Monitor Bounce Rates

Hard bounces (invalid addresses) hurt your reputation. Remove them immediately.

Soft bounces (temporary issues) can be retried, but repeated soft bounces should be investigated.

Target: Keep overall bounce rate under 2%.

Handle Spam Complaints

Every spam complaint damages your reputation. Target under 0.1% complaint rate.

Reduce complaints by:

• Only emailing people who opted in
• Making unsubscribe visible and easy
• Sending relevant content

Content Best Practices

Avoid Spam Triggers

Common spam triggers:

• ALL CAPS SUBJECT LINES
• Excessive punctuation!!!
• Spammy words: "FREE", "ACT NOW", "LIMITED TIME"
• URL shorteners (bit.ly, etc.)
• Too many images, too little text

Use Plain Text Fallbacks

Always include a plain text version alongside HTML:

await client.send({
  from: 'hello@yourapp.com',
  to: ['user@example.com'],
  subject: 'Your Weekly Summary',
  body: htmlContent,
  text: plainTextContent // Always include this
})

Proper HTML Structure

• Include DOCTYPE declaration
• Use table-based layouts for compatibility
• Inline CSS styles
• Keep total email size under 102KB

List Hygiene

Double Opt-In

Require confirmation before adding to your list. This ensures:

• Valid email addresses
• Confirmed consent
• Higher engagement rates

Regular Cleaning

Remove inactive subscribers who haven't engaged in 6+ months. They hurt your engagement metrics and deliverability.

Sunset Policies

Create automatic policies to handle unengaged users:

1. User hasn't opened in 3 months → Send re-engagement email
2. Still no engagement after re-engagement → Move to suppression list

Monitoring and Alerts

Key Metrics to Track

Use Webhooks

Set up real-time notifications for critical events:

// Configure webhooks in your dashboard
// Or handle events programmatically

client.onReceive({
  onBounced: (event) => {
    // Remove from active list
    removeFromList(event.recipientEmail)
  },
  onComplained: (event) => {
    // Immediately suppress
    addToSuppressionList(event.recipientEmail)
  }
})

Testing Before Sending

Email Preview Tools

Test your emails across different clients:

• Desktop: Outlook, Apple Mail, Thunderbird
• Web: Gmail, Yahoo, Outlook.com
• Mobile: iOS Mail, Gmail app

Spam Score Testing

Send to spam testing tools before launching campaigns to check your spam score.

Seed Lists

Send to seed addresses at major ISPs to verify inbox placement.

Conclusion

Email deliverability is technical but manageable. Focus on:

1. Authentication — SPF, DKIM, DMARC properly configured
2. Reputation — Monitor bounces and complaints
3. Content — Avoid spam triggers
4. Hygiene — Clean lists regularly

With SimpleEmailAPI, we handle many of these concerns automatically — including authentication setup, bounce handling, and reputation monitoring. You focus on your product; we focus on getting your emails delivered.