Email Deliverability Guide for Developers

What is email deliverability?

Email deliverability measures whether your emails reach the recipient's inbox. It's not the same as delivery rate — an email can be "delivered" to the spam folder. True deliverability means inbox placement.

Four factors determine deliverability:

1. Authentication — proving you're authorized to send from your domain
2. Reputation — your track record as a sender (domain and IP)
3. Content — what's in your email and how it's formatted
4. Engagement — how recipients interact with your emails

Email authentication: SPF, DKIM, and DMARC

Authentication is the foundation of deliverability. Without it, ISPs have no way to verify you're a legitimate sender.

SPF (Sender Policy Framework)

SPF is a DNS TXT record that lists which servers are authorized to send email for your domain. When a receiving server gets an email from your domain, it checks SPF to verify the sending server is on the list.

DKIM (DomainKeys Identified Mail)

DKIM adds a cryptographic signature to every outgoing email. The receiving server uses your public key (published in DNS) to verify the signature, confirming the email wasn't altered in transit.

DMARC (Domain-based Message Authentication)

DMARC ties SPF and DKIM together and tells ISPs what to do when authentication fails. It also provides reporting so you can monitor authentication results.

For step-by-step setup instructions, see our SPF, DKIM & DMARC documentation.

Sender reputation

ISPs assign reputation scores to both your domain and your sending IP address. High reputation means inbox placement. Low reputation means spam folder or outright rejection.

Reputation is built over time through:

• Low bounce rates (below 2%)
• Low complaint rates (below 0.1%)
• Consistent sending volume
• Positive recipient engagement (opens, replies)
• Proper authentication (SPF, DKIM, DMARC passing)

Monitor your reputation with Google Postmaster Tools and RelayPost's public deliverability dashboard.

Domain warmup

New domains have no sending history, which means no reputation. ISPs are cautious about new senders. Domain warmup is the process of gradually increasing your sending volume over 4-8 weeks to build positive reputation.

A typical warmup schedule:

• Week 1: 50-100 emails/day to your most engaged recipients
• Week 2: 200-500 emails/day
• Week 3: 500-1,000 emails/day
• Week 4+: Double weekly until you reach your target volume

During warmup, only send to recipients who are likely to open and engage. Bounces and complaints during warmup are especially damaging.

Content best practices

• Use a recognizable From name and address
• Write clear, relevant subject lines — avoid ALL CAPS and excessive punctuation
• Maintain a good text-to-HTML ratio — don't send image-only emails
• Include a plain-text version alongside HTML
• Always include an unsubscribe mechanism
• Avoid URL shorteners — use full URLs
• Don't use spam trigger phrases ("act now", "free money", "click here")

Monitoring deliverability

Track these metrics continuously:

• Inbox placement rate — percentage of emails reaching the inbox (target: 95%+)
• Bounce rate — percentage of emails rejected (target: below 2%)
• Complaint rate — percentage of recipients marking as spam (target: below 0.1%)
• Open rate — while not a direct deliverability metric, declining open rates can signal spam placement

RelayPost provides real-time delivery tracking via webhooks and a public deliverability dashboard showing aggregate inbox placement rates by ISP.

Related guides

• Emails Going to Spam? Here's How to Fix It
• Transactional Email Best Practices
• SPF, DKIM & DMARC Setup Guide
• Domain Verification

Frequently asked questions