Definition
Email deliverability debugging is the structured investigation of why messages are not reaching the inbox. Rather than guessing at the cause, debugging follows the delivery path layer by layer: checking DNS authentication records, examining SMTP server logs, reviewing bounce codes, testing inbox placement with seed addresses and analysing engagement and complaint data.
Effective debugging treats deliverability as a chain of dependencies. A failure at any link — a missing SPF record, a blocklisted IP, content that triggers spam filters — can prevent inbox placement regardless of how strong the other links are.
The Debugging Checklist
| Layer | What to Check |
|---|---|
| Authentication | Are SPF, DKIM, and DMARC records correctly configured and passing? |
| IP Reputation | Is your sending IP listed on any blocklists? |
| Domain Reputation | Have you checked Google Postmaster Tools and Microsoft SNDS? |
| Content | Are there spam trigger words, broken images or excessive link volume? |
| Engagement | Are subscribers opening, clicking and marking as not spam? |
| Infrastructure | Are you sending from a warmed IP with consistent volume? |
Best Practices
- Debug from the top of the stack down — authentication first, then infrastructure, then content
- Use a reproducible test: send to your own seed addresses at Gmail, Yahoo and Outlook and observe placement
- Compare current delivery data against a known-good baseline period to isolate when the issue began
- Document each debugging session so recurring issues can be resolved faster in the future
Related Glossary Terms
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
DMARC Alignment
DMARC identifier alignment determines whether the domain in the From header matches the domains used in SPF and DKIM authentication. Strict or relaxed.
Acceptance Rate
Acceptance rate is the percentage of emails submitted for delivery that are accepted by the recipient's mail server, representing the first deliverability checkpoint before inbox placement is determined.
Email Acceptance Rate
Email acceptance rate measures the percentage of emails accepted by recipient mail servers versus total sent. It is a core deliverability metric that reveals whether your sending infrastructure is trusted by receiving servers.
Email Account Health Score
A composite metric that evaluates the overall health of an email sending programme based on deliverability, engagement, list quality, and compliance factors.
Email Active Subscriber
An active email subscriber has opened or clicked an email within a defined recency period, typically 30-90 days by industry. Active subscriber rate of 40-60% is typical for healthy email lists.