When email performance drops, most brands check the obvious suspects. Subject line, send time, list quality, segmentation. But there's a cause that gets missed far more than it should: the design of the email itself.
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Deliverability is not just a strategy problem
Deliverability is whether your email actually arrives in the recipient's inbox, as opposed to landing in spam, getting clipped, or failing to render properly across clients.
Most founders think about sender reputation, list hygiene, content flags. Those matter, but the technical build of the email (the HTML weight, the code structure, how assets are embedded) also affects how email clients process it. Most people file it under technical. The design side tends to get ignored.
What causes Klaviyo emails to clip in Gmail
Gmail clips emails that exceed a certain HTML size threshold, currently around 102KB. When that happens, the customer sees a cut-off version with a 'View entire message' link at the bottom. Most of them won't click through to read the rest.
Clipping affects click rates and conversion rates. It also signals to Gmail that you regularly send heavy emails, which can quietly affect long-term inbox placement without much notice.
HTML weight comes from the volume of inline CSS, image sizes, redundant code and, less obviously, assets embedded in the HTML that aren't visible in the rendered email at all. Invisible in the email, but present in the HTML and Gmail is counting every byte.
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How email design affects deliverability
When an email is designed by someone unfamiliar with how email clients handle HTML, the code tends to end up heavier than it needs to be. Templates built on top of other templates carry legacy elements forward. Styling rules get duplicated. Images sit in the HTML without appearing on screen.
You won't see any of this in a standard preview. It only surfaces when you go looking at the source code, or when Gmail starts flagging things.
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What we found in a recent client audit
When we started working with a new Klaviyo retainer client, their emails were being flagged as possibly clipping. On inspection, there was a logo embedded in the source code of the header which was invisible in the rendered email, sitting quietly in the HTML, adding weight to every single send.
The previous design had been built without a real understanding of how Klaviyo handles HTML in practice. The email looked completely normal. Under the hood it was carrying weight it had no reason to carry, and was probably reaching fewer inboxes than it should have.
We rebuilt the header and footer from scratch, keeping only what was needed. The result was a significantly lighter email, and we'd expect to see improved deliverability over the sends that follow.
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What good Klaviyo email design looks like technically
Deliverability has to be part of the brief from the start. That means building natively in Klaviyo rather than importing from tools that layer in unnecessary code. Keeping inline CSS consistent so rules aren't being duplicated across elements. Removing anything that's present in the HTML but not visible in the email; if it's not showing, it shouldn't be there. Optimising image sizes before upload and checking HTML weight regularly using Klaviyo's preview tools or an external testing tool.
The technical bar isn't high but it does require someone who thinks about how email clients actually process HTML, not just how an email looks on screen.
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How to check if your email design is causing deliverability issues
In Klaviyo, check your campaign send reports for deliverability warnings Gmail flagging potential clipping will show up there. You can also test HTML weight by sending yourself a test and inspecting the source code size.
If open rates have dropped without an obvious strategic cause, or customers have mentioned emails looking broken or cut off, the design is worth looking at.
Next Steps
If you want to understand whether your email design is affecting deliverability, find out more about our Email Design services or get in touch to talk through what you're seeing.
