Email Deliverability: What It Is, What Breaks It, and How to Fix It

Learning to fix and improve email deliverability in Klaviyo starts with understanding what deliverability actually is. Deliverability is the most invisible, most underestimated, and most catastrophic problem in email marketing. You can send every campaign, run every flow, and write compelling copy — and if you have a deliverability problem, none of it reaches anyone.

What deliverability actually is

Deliverability is not binary. It is a spectrum from consistently landing in the primary inbox for the vast majority of recipients to being filtered to spam for most. Deliverability is not binary. It is a spectrum from consistently landing in the primary inbox for the vast majority of subscribers all the way to being blocked entirely by certain inbox providers. Gmail and Apple Mail together handle the vast majority of consumer email. Both use engagement data to decide how to route your emails: open rates, click rates, spam complaint rates, positive actions like replying or moving emails from spam to inbox, and unsubscribes.

What breaks deliverability

Sending to unengaged subscribers

Every time you send to someone who never opens, that non-engagement is logged. Enough non-engagements and your sender reputation declines — not just for that subscriber, but for everyone you send to. This is the most common cause of deliverability problems. Businesses accumulate large lists without regularly suppressing people who have stopped engaging.

Spam complaint rates above 0.1 percent

A spam complaint is stronger than an unsubscribe — it tells inbox providers this email has no place in my inbox. Gmail has published this threshold publicly. Above 0.1 percent complaint rate and inbox placement issues begin. Above 0.3 percent is a genuine crisis.

Sudden volume spikes and missing authentication

If you normally send 1,000 emails per week and suddenly send 50,000, inbox providers flag it as suspicious. Separately, SPF, DKIM, and DMARC are the technical records that tell inbox providers your emails are legitimate. Without these configured correctly, your emails look suspicious. Most email platforms help you set them up but they are frequently misconfigured by default.

How to check your deliverability right now

Go to postmaster.google.com and verify your sending domain. This gives you direct visibility into how Gmail treats your emails: domain reputation, IP reputation, and spam complaint rates. If your domain reputation shows Bad or your spam rate exceeds 0.1 percent, you have a problem requiring immediate attention. Also pull open rates segmented by email client in Klaviyo — if Gmail open rates are more than 10 percentage points below your overall rate, Gmail is filtering for a meaningful percentage of subscribers.

Run your free FlowScore audit at klaviyoscore.com to see your deliverability score and what is affecting it.

The fix if deliverability is broken

• Stop sending large campaigns until you have addressed the root cause.
• Run a re-engagement sequence for anyone who has not opened in 90 days. Two emails designed to either re-engage or self-select out. Suppress anyone who does not engage.
• Remove hard bounces from your list immediately.
• Fix SPF, DKIM, and DMARC authentication records.
• Identify what is generating spam complaints and stop sending it.

Recovery takes four to six weeks of clean sending. The rebuild starts the moment you stop the behaviors that broke it.

FlowScore audits your Klaviyo account for deliverability issues including suppression rate, list health, and sending consistency — free at klaviyoscore.com.

Need more tools and frameworks? Browse everything at signifystudio.com/library.

---