You ran a campaign last Tuesday. Open rate was 28%. You ran a similar campaign this Tuesday. Open rate was 19%.
Same list. Same kind of subject line. Same send time. What happened?
Klaviyo will show you the drop. It will not tell you why. And in a lot of cases, the answer has nothing to do with your subject line, your segment, or your offer. It's that some percentage of your sends stopped landing in the inbox.
That's a sender reputation problem. And it's one of those things that's invisible until it's already costing you money.
What sender reputation actually is
Every mailbox provider (Gmail, Yahoo, Outlook, Apple) keeps a running score on you. Not on your email, on you. The sending domain. The sending IP. The patterns of who opens, who clicks, who marks spam, who deletes without reading.
When that score is healthy, your emails go to the primary inbox. When it slips, they start going to Promotions. When it gets worse, they go to spam. When it gets really bad, they get rejected at the door.
The thing is, this happens gradually. There's no alert. Klaviyo doesn't have a banner that says "hey, Gmail thinks you're getting spammy." You just notice that your numbers are softer than they used to be, and you blame your subject lines.
Why Klaviyo can't see this
Klaviyo tracks what it can see. It sees that the email was sent. It sees that the pixel fired, which means open. It sees that a link was clicked. It does not see whether the email landed in the inbox or in spam. The pixel fires either way (if the person opens it). Most people don't open emails in their spam folder.
So when your inbox placement drops, you see it as a lower open rate. Not as "20% of your sends went to spam." Klaviyo has no way of knowing that.
This is the core of the problem. The tool you use to send email is also the tool you use to measure email. And it's measuring the wrong thing for this question.
The early warning signs
There are things you can watch for before the open rate drop shows up clearly:
Unsubscribes start coming faster.Not always more total, but earlier in the send cycle. People who are seeing your emails are reacting more quickly because the email got more aggressive about asking for attention (or they were only seeing you sporadically and now they're seeing you again).
Your Gmail engagement drops first.Gmail tends to be the canary. If your Gmail open rate is sliding while Yahoo and Apple Mail stay flat, that's a signal.
Click rates hold steady while opens drop.This is a big one. If the people who do open are clicking at normal rates, the problem isn't your content. It's that fewer people are seeing it.
Your engaged segment shrinks.Look at "opened in last 30 days" as a count, not a percentage. If that number is sliding while your list is growing, your reach is contracting.
What actually hurts your reputation
Most reputation damage comes from a few sources, and they're usually self-inflicted:
Sending to people who never engage. Klaviyo will happily send to everyone on your list every time, but mailbox providers see "you keep emailing this person and they never open it" and they flag you. Cold subscribers don't just not buy. They actively damage your ability to reach the people who would.
Big list adds without warm-up. If you imported a list, ran a giveaway, or did a popup campaign that doubled your list in two weeks, your sender reputation has to absorb that. New subscribers haven't engaged yet, and from Gmail's perspective, you suddenly look like you're emailing a lot of unfamiliar addresses.
Spam complaints. One in a thousand is fine. One in five hundred is a problem. Most brands don't track this number because Klaviyo doesn't put it front and center.
Sending too often after a quiet period. If you go from sending once a week to sending five times a week because Black Friday is coming, the spike itself raises flags. The frequency change matters as much as the absolute frequency.
How to actually check
Here's the thing nobody tells you. Klaviyo gives you the symptoms. To diagnose the cause, you need to look outside Klaviyo.
The free tools to start with:
Google Postmaster Tools.This is a Google product that shows you your sender reputation from Gmail's perspective. Domain reputation, IP reputation, spam rate, authentication rates. You set it up once with a DNS record. It is the single most useful free tool for deliverability and almost nobody uses it.
Seed lists.Services like GlockApps or Inbox Monster send your campaign to a set of dummy inboxes at major providers and tell you whether you landed in inbox, promotions, or spam at each one. You don't need to run this on every campaign. You need to run it once a month so you have a baseline.
Your own test addresses.Have a personal Gmail, a Yahoo, an Outlook, and an iCloud account that you subscribe yourself to. Open them after each send. Where did your email land? This isn't scientific but it's a free smoke test.
What to do when you find a problem
Sunset your cold subscribers. This is the hardest thing to do because the number shrinks. But the people who haven't opened in 90 days are not going to buy. They're just dragging down your reputation. Remove them or move them to a "win back, then sunset" flow.
Slow down. If you're sending five times a week and your reputation is dropping, send three times a week to a more engaged segment for two weeks. Let the score recover.
Tighten the top of the funnel. Where are new subscribers coming from? If a lot of them are from a giveaway or a popup that doesn't filter for intent, you're letting in addresses that will never engage. Add a double opt-in. Make the popup ask for more than just an email.
Authenticate everything. SPF, DKIM, DMARC. If you don't know what these are, ask whoever set up your domain. Klaviyo's setup guide covers it. Skipping this stuff in 2026 is asking for trouble.
The bigger point
Email marketing has this weird gap where the platform you use is also the scoreboard, and the scoreboard doesn't show the most important number. Inbox placement is where your revenue actually lives. Open rate is just the proxy you can see.
If your numbers are sliding and you can't figure out why, don't assume your content got worse. Check whether your sends are landing first. The answer might not be in Klaviyo at all.
This is part of why we built Cadento around the idea that planning your sends matters as much as sending them. Sending less to people who engage more is better for your reputation, your numbers, and your customers. The platform won't push you toward that. You have to push yourself.