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Inbox Placement Testing and Deliverability: What Marketers Need to Know

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RE:markable is the weekly email about emails. Dropping the latest email marketing news, updates, insights, free resources, upcoming masterclasses, webinars, and of course, a little inbox mischief.

 

 

Does anything trigger fight or flight for marketers and email specialists quite like an inbox placement test telling you that half of your emails are flying straight to the spam folder?

You’re not alone.

Inbox placement testing and deliverability are one of those things that every marketer hears about, but few fully understand.

Here’s the real kicker, though – used right, inbox placement testing is an incredible tool. Used wrongly, it’ll give you a heart attack, zero clarity, and no future incentive to use it effectively as part of your wider email marketing strategy.

So let’s work it out together.

 

What is inbox placement testing?

Inbox placement testing is your sneak peek into how mailbox providers treat your emails.

Instead of you sitting and waiting for signs that your emails are going unread, or for a subscriber to say they “never saw your newsletter” (translation: it’s hiding in spam), you send your campaign to a seed list.

“But what’s a seed list?” you say? 

A seed list is hundreds of test inboxes across different providers (Gmail, Outlook, Yahoo, Apple, corporate filters like Mimecast or Proofpoint). The inbox placement tool monitors where your email lands:

  • Inbox
  • Spam/junk
  • Promotions tab (for Gmail)
  • Or whether it’s blocked altogether

Tools like ZeroBounce, GlockApps, Validity, Email on Acid, Warmy, and more all work off the same principle: they’ve built panels of seed accounts to show you a “placement map.”

It sounds pretty great, right? But (and you probably sensed a but coming), there are limitations.


 

Inbox placement ≠ deliverability 

Inbox placement testing and deliverability might sound like one and the same, but that’s a massive misconception.

Inbox placement testing does not equal full deliverability testing.

Here’s what deliverability is influenced by:

  • Your sender reputation (IP, domain, historical performance)
  • Engagement signals (opens, clicks, replies, complaints, unsubscribes)
  • Authentication (SPF, DKIM, DMARC, BIMI)
  • Your sending patterns and data quality
  • And dozens of micro-signals unique to each provider (as if things weren’t difficult enough, eh?)

Inbox placement tools really are only one slice of the cake because they’re telling you where one campaign landed in one moment in time, across a handful of seed accounts.

In short, inbox placement tools are a useful indicator… but they aren’t the truth, the whole truth, and nothing but the truth.

 

 

Why inbox placement testing can “trigger” spam filters

I hope you’ve got your glass of milk because this is where things can get spicy. 

Unfortunately, inbox placement testing itself can appear a little sketchy and suspicious to mailbox providers.

That’s because:

1. Seed accounts behave differently

In the same way that a bouncer is looking out for excessively drunk behaviour to turn someone away, ISPs are looking for unusual behaviour to treat as a red flag too.

The issue is that most inboxes don’t behave like real subscribers. Which means that they don’t open, click, reply, or move emails around – a dormant inbox receiving sudden volume raises alarms.

2. Corporate filters are brutal

Yep, another case of being turned away at the door immediately. If you send to a block of Proofpoint, Mimecast, or Barracuda test accounts, the filter is likely to think, “Who is this sender suddenly blasting us?” and shove you straight into spam.

3. One bad signal can snowball

If your authentication isn’t rock solid, your content looks risky, or you’re shaky in other areas of your email marketing, an inbox placement test can exaggerate the problem because seed inboxes don’t have a positive engagement history to balance things out.

That’s why running a single test (or ten, or even thirty) isn’t enough to ‘prove’ anything (or to lose hair over, because quite frankly, it’s stressful).



 

The right way to use inbox placement tests 

When I run deliverability audits, I don’t just fire off one placement test and call it a day.

Instead, I focus on inbox placement testing and deliverability more holistically to see the bigger picture. I:

  • Use four different tools (because each seed panel is built differently)
  • Run tests for 30 days across multiple campaigns
  • Compare placement against real subscriber data (engagement, bounces, complaints, inbox behaviour)
The reason why this is so much more helpful is that instead of seeing one-off results that make you panic (and potentially pivot your entire approach for no good reason), you’re able to see patterns based on a combination of useful data.


 

How to interpret results (without losing your mind)

In short, don’t panic and assume the worst.

Instead:

  • Validate bad results: If one tool says “80% spam,” don’t assume the worst. Check the same campaign across other panels and compare to your real open/click data – it’s better to interpret results against data than in a vacuum.
  • Validate good results too: “100% inbox” might look and feel lovely, and I don’t want to burst your bubble, but if your Gmail engagement is tanking, something’s off.
  • Ask the provider hard questions: How often do they refresh seed accounts? Do those inboxes show any activity? Are they balanced across regions and providers?
  • Remember the context: If your domain is brand new or you’re warming up an IP, expect skewed results.
And above all else, never take anything regarding inbox placement testing and deliverability as your one source of truth. Validate, validate, validate!

 

So, can inbox placement tests trigger filters?

Technically, yes. Especially on corporate filters where a sudden burst to inactive inboxes looks dodgy.

Really, the bigger risk is misinterpreting the results and either ignoring or not recognising a real problem, or freaking out about a fake one.

It’s all about using a bigger picture, holistic approach. 

The TL;DR:

  • Inbox placement testing is a useful indicator, not a deliverability silver bullet (it might flag a symptom but not the root issue).
  • Yes, it can trigger filters, but more often it gives you a skewed view without the context you need to improve and adjust your approach.
  • Validate results across tools, over time, and alongside real data for actionable insights rather than panicked frights.
  • If you don’t want to waste 30 days on testing, that’s what my deliverability audits are for. 


Final word: don’t go it alone

Inbox placement testing is kind of like checking your car’s dashboard… it can be useful, but it won’t diagnose why the engine light has been flashing.

You need a full diagnosis! That’s where my deliverability audits come in. They pull together:

  • Multiple placement tools (so you get averages, not outliers)
  • Authentication checks
  • Domain and IP reputation analysis
  • Real-world engagement and postmaster data
  • 30-day pattern testing
The result? You save a ton of wasted time, learn what’s really happening (not just what one test says), and walk away with a clear action plan that includes fixes, next steps, and how to prevent issues in the future.

If you’re struggling with deliverability issues or think you might need some peace of mind before peak season, get in touch here to find out more about my deliverability audits.

(Pssst - they’ve been selling like hotcakes because the difference between ‘inbox’ and ‘spam’ is the difference between growth and ghosting!)

 

 

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RE:markable is the weekly email about emails. Dropping the latest email marketing news, updates, insights, free resources, upcoming masterclasses, webinars, and of course, a little inbox mischief.
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