Email deliverability test: how to check inbox vs spam placement

This guide walks you through how to run a seed test in Klaviyo that shows you exactly where your emails land across major mailbox providers, how to interpret the results, and what to fix based on the patterns you see.

SO your Klaviyo campaign shows a 98% delivery rate, but your revenue is down 40%. The problem isn't your offer or your subject line—it's that most of your emails are landing in spam, and your delivery rate can't tell you that. Let’s get into what you’re going to do now.

What seed testing actually tells you v.s. what Klaviyo's metrics can

Email deliverability refers to where your email lands after the recipient's mail server accepts it. Your email can land in the inbox, the spam folder, a promotions tab, or go missing entirely. This is different from your delivery rate, which only tells you whether the server accepted your email without bouncing it back.

Here's why that difference matters. Klaviyo's delivery rate doesn't show you where your emails actually land. You might see a 98% delivery rate and think everything's working, but 60% of your Gmail subscribers could be finding your message in spam instead of their inbox.

A seed test fills that gap. You send a campaign to a small list of test email addresses across major mailbox providers like Gmail, Yahoo, Outlook, and Apple Mail. Then you manually check where each email lands. It's the only way to see inbox placement with your own eyes, provider by provider.

Privacy changes have made this even more important. Apple Mail Privacy Protection (MPP) and Gmail's image proxy pre-load images automatically, which inflates your open rates. You can't trust opens as a stand-in for inbox placement anymore. A seed test gives you the real answer.

Here's what this looks like in practice. An ecommerce brand ran a Black Friday Cyber Monday preview campaign with a 97% delivery rate and 18% open rate. The team thought their subject line was weak. After running a seed test, they found 60% of Gmail addresses landed in spam. Without the test, they would've changed the wrong thing.

Build your seed list—which providers to include and how many addresses you need

You can set up a working seed list in about 20 minutes. Start with 15–20 email addresses spread across 4–5 major mailbox providers, weighted toward where your subscribers actually check their email.

Here's a simple breakdown:

• Gmail (personal): 4–5 addresses. Gmail is the largest consumer email provider and filters promotional content aggressively.
• Yahoo: 2–3 addresses. Yahoo shares infrastructure with AOL and has strict authentication requirements.
• Outlook/Hotmail: 2–3 addresses. Microsoft's consumer email service uses its own filtering logic.
• Apple Mail (iCloud): 2–3 addresses. This is growing in importance, especially for mobile-first brands.
• AOL: 1–2 addresses if you have a meaningful subscriber base there.

You can create accounts for free using personal email addresses, or use a paid seed testing tool like GlockApps, MailGenius, or 250ok. Klaviyo also offers built-in inbox testing capabilities through Mailgun integration. Free works fine for most B2C brands. You just want addresses you can access quickly.

Store your seed list as a dedicated segment or list in Klaviyo. Name it something clear like "Seed Test List" and tag it so you never accidentally suppress or delete it. You'll use this same list repeatedly, so treat it like part of your infrastructure.

One important note: don't use role-based addresses like info@ or support@. Don't use addresses that have never engaged with your sending domain. You want addresses that behave like normal consumer accounts that receive mail regularly and mirror actual subscriber behavior.

Run the test in Klaviyo—campaign setup, tracking, and what to keep consistent

The key to a useful seed test is matching your real send as closely as possible. If your test doesn't mirror the actual campaign, your results won't predict what happens when you send to your full list.

Start by creating a campaign in Klaviyo. Don't use a flow, since you want full control over timing and content. Use the exact same setup as your planned send: same from-name and from-address, same sending domain, same content (subject line, preheader, body copy, images, links), and same send time or as close as possible. Mailbox provider filters can change throughout the day, so timing matters.

Send the campaign to your seed list segment. Wait 10–15 minutes before checking results. Mailbox providers typically process mail quickly, but give it a few minutes to settle.

Check each inbox manually. Look in the primary inbox first, then the Promotions tab for Gmail, then the spam or junk folder. If you don't see the email anywhere, search for the subject line. Sometimes emails get filtered in ways that aren't immediately obvious.

Record your results in a simple table with columns for provider, inbox, spam, and missing. A spreadsheet works fine. You don't need fancy tracking.

Klaviyo-specific tips: Turn off Smart Sending for seed tests so you're not accidentally suppressing test addresses based on engagement history. Double-check your tracking settings, especially if you're testing a plain-text version. Make sure your real send matches. Run your seed test at least 24 hours before your real campaign so you have time to fix issues if they come up.

How to score your results—inbox vs. spam vs. missing, and what patterns usually mean

Create a simple scorecard to make sense of your results. Calculate inbox rate by provider: divide the number that landed in the inbox by the total number you sent to that provider, then multiply by 100. Do the same for spam rate. Track anything that didn't appear anywhere as "missing." It's rare, but it happens.

The patterns you see tell you what's broken. Here's what to look for:

• Gmail-only spam placement: This usually points to a content or engagement issue. Gmail filters promotional content aggressively through its tabbed inbox system, so you might be using language that triggers filters, you have broken links, or your Gmail subscribers have low engagement history with your domain.
• Yahoo or AOL spam placement: This typically indicates an authentication or reputation problem. Double-check that your DMARC alignment is correct and review your recent complaint rates in Klaviyo's campaign analytics.
• Mixed spam across all providers: This suggests a list quality or cadence problem. You might be sending to unengaged subscribers, or you've ramped your volume too quickly and damaged your sender reputation.
• High missing rate: This points to DNS or authentication failure, or you're on a blocklist. Check that your SPF, DKIM, and DMARC records are configured correctly in Klaviyo at Settings > Email > Sending, and run a blocklist check using a tool like MXToolbox.
• 100% inbox placement but low open rates: This means you have a measurement problem, not a deliverability problem. This is usually Apple MPP or engagement decay, which is frustrating but not a reason to panic or change your approach.

What does "good" look like? You want 85%+ inbox placement across providers. Anything below 70% demands immediate attention before you send to your full list.

Common false alarms—deliverability vs. engagement vs. privacy shifts

Not every metric drop is a deliverability crisis. Here's how to tell the difference between a real problem and measurement noise.

False alarm #1: Your open rates dropped 15% month-over-month, but your seed test shows 90%+ inbox placement. This is a tracking or privacy issue like Apple MPP, image blocking, or Gmail's image proxy, not deliverability. Focus on click rates and revenue instead of opens.

False alarm #2: Engagement dropped after you changed your segmentation. You changed your audience, not your deliverability. Compare the same segment over the same time period, or you're measuring two different things.

False alarm #3: One campaign underperformed compared to your average. This could be subject line, send time, offer fatigue, or audience saturation. Run a seed test on your next campaign before you assume it's deliverability.

Here's the rule: if your seed test shows spam placement and your revenue is down, you have a real deliverability problem. If your seed test shows inbox placement and your opens are down, you have a measurement or engagement problem. The seed test is your tiebreaker.

What to do next—the highest-impact fixes based on your result pattern

Once you know what's broken, here's how to fix it.

If Gmail is sending you to spam

Tighten your segmentation immediately. Send only to Gmail subscribers who opened or clicked in the last 30–60 days. This rebuilds engagement signals and tells Gmail's filters that your mail is wanted.

Review your content for spam triggers. Remove excessive links (more than 3–4 in a single email), all-caps text in subject lines or body copy, and misleading subject lines that don't match your content.

Slow your cadence if you've increased frequency recently. Pull back for 7–14 days, then ramp up gradually while monitoring inbox placement.

If Yahoo or AOL is sending you to spam

Validate your DMARC alignment first. Go to Settings > Email > Sending in Klaviyo and confirm your DNS records are configured correctly. Yahoo is strict about authentication. If your DMARC policy isn't aligned, you'll land in spam.

Review your complaint rates in Klaviyo. Go to Campaigns > Analytics > Metrics and look at spam complaint rates over the last 30 days. If complaints are elevated, pause sends to unengaged Yahoo and AOL subscribers.

If you're seeing spam across all providers

Audit your list source. Where did your newest subscribers come from in the last 30 days? If you've added a new lead source like a giveaway, a purchased list, or a co-marketing campaign, that's likely the culprit.

Implement a re-engagement flow or sunset policy. Stop sending to subscribers who haven't engaged in 90+ days. This improves your engagement rate and signals to mailbox providers that you're sending to people who want your mail.

Check your sending domain reputation using Google Postmaster Tools or Microsoft SNDS. If your reputation is damaged, you'll see it reflected in those tools, and you'll know you have to rebuild slowly.

If emails are missing entirely

Validate your SPF, DKIM, and DMARC records in Klaviyo at Settings > Email > Sending. If any of those are misconfigured, some mailbox providers will reject your mail entirely.

Run a blocklist check using MXToolbox or MultiRBL. If you're on a blocklist, you'll typically see high missing rates because mail servers are rejecting your messages before they reach the inbox.

Contact Klaviyo support if your authentication looks correct and you're not on any blocklists. There may be an issue with your sending infrastructure that requires escalation.

What to do right now

Here's your action plan:

1. Build your seed list today: 15–20 addresses across Gmail, Yahoo, Outlook, and Apple Mail.
2. Save it as a dedicated segment in Klaviyo so it's ready when you need it.
3. Run a seed test on your next campaign at least 24 hours before you send to your full list.
4. Record results in a simple table and compare inbox placement by provider.
5. If you see spam placement above 30% on any single provider, pause and fix the issue before sending to your full list.

Make this a habit. Run a seed test before every major promo, after any domain or authentication change, and anytime your campaign metrics drop unexpectedly. The 20 minutes you spend testing can save you from a revenue miss that takes weeks to recover from.