Common Email Rendering Issues and How to Fix Them

Every email developer has a version of the same story. The campaign looked perfect in preview, passed internal review, and went out to the full list. Then the screenshots started coming in: broken layouts in Outlook, content cut off mid-sentence in Gmail, logos that vanished the moment someone opened the email in dark mode.

The frustrating part is that these failures are entirely predictable. The same rendering issues show up in the same email clients for the same reasons, and they have for years. Fixes exist for most of them. The challenge is that nobody collects all the fixes in one place, and the ones that do exist are often version-specific and fragile. So teams end up rediscovering the same workarounds every time something breaks.

Why emails render differently across clients

Web browsers converged on shared standards years ago. Email clients never did. Chrome, Firefox, and Safari all interpret HTML and CSS in broadly consistent ways because they share modern rendering engines. Email clients use at least five distinct approaches, and each one has its own opinion about what your code should look like.

The differences come down to rendering engines:

Apple Mail accounts for roughly 57% of email opens, followed by Gmail at around 30% and Outlook at 4%. Those global numbers obscure the enterprise reality. Outlook has 400 million users and 60% of Fortune 500 companies use it as their primary email system. If your audience is enterprise B2B, Outlook is not a 4% problem.

The result is that the same HTML file can look completely different depending on where it opens, not just a shifted pixel or a slightly different font, but structurally different layouts with missing elements and invisible calls to action.

Outlook and the Word rendering engine

Lauren Meyer, who has worked in email development for more than two decades, puts it simply: "It's Outlook. Since forever. Full stop."

The core issue traces back to 2007, when Microsoft replaced Internet Explorer with Microsoft Word as Outlook's HTML rendering engine. Word. The application you use to write documents. That decision has persisted across every desktop version since (2010, 2013, 2016, 2019, 2021, and Microsoft 365 for Windows all use it), and it was never designed to render modern HTML email.

The specific limitations are well documented, and worth understanding because they dictate how enterprise email teams have to build (whether they want to or not):

• No CSS background images. Developers have to use Vector Markup Language (VML), an obsolete markup format that exists nowhere else in modern web development, to get background images working in Outlook.
• Inconsistent padding and margins. Outlook ignores or misinterprets CSS padding in most situations. The workaround is placing padding inside table cells rather than using CSS properties, which forces table-based layouts over semantic HTML.
• Div tags are effectively ignored. Outlook does not support width or height styling on <div> elements. They default to the height of their text content and 100% width, regardless of CSS. Everything goes into tables.
• Font fallback breaks. When Outlook encounters a custom web font, it does not fall back to the next font in the stack. It skips the entire fallback chain and defaults to Times New Roman (yes, really). Developers need MSO-specific CSS properties to control font behavior.

And then there are the bugs that have persisted for nearly two decades without fixes. The white lines problem is the most notorious: random horizontal lines appear in emails at certain display scaling levels, and it has been documented since Outlook 2007. The workarounds involve switching font sizes from odd to even pixel values, adjusting table cell heights, and adding empty conditional line breaks targeted specifically at Outlook. None of these address the root cause. They are trial-and-error patches for a bug Microsoft has never prioritized fixing.

"There are just too many issues across too many versions," Meyer notes. "White lines, padding quirks, GIF fallbacks, background images with VML, font fallbacks that suddenly turn into Times New Roman. And none of these versions are going away anytime soon. They're still heavily used in corporate environments where laptops get replaced somewhere between 'every two years' and 'end of civilization.'"

Microsoft has built a new Outlook for Windows using Chromium-based rendering (WebView2), which brings it in line with modern web standards. But the new client lacks features that enterprise IT depends on, including VBA macro support, COM add-ins, and PST file support, which means classic Outlook with its Word engine will remain the enterprise default through at least 2027.

Gmail clipping and the 102KB threshold

Gmail imposes a hard limit: any email whose HTML exceeds 102KB gets clipped. That is 102KB of code, not the total file size including images (images are loaded separately, though heavy image use can bloat your HTML if the template structure gets complex). Gmail shows the first 102KB and adds a "[Message clipped] View entire message" link at the bottom. Everything below that cutoff disappears from the recipient's view.

The consequences go well beyond aesthetics. When an email gets clipped, any CTAs, unsubscribe links, or tracking pixels below the threshold become invisible. So your engagement numbers look worse than they should (tracking pixels never fire, making opened emails appear unopened), and if your unsubscribe link gets hidden, recipients who want to opt out are more likely to hit the spam button instead.

The 102KB limit is easier to hit than most teams expect:

• Editor-generated code from ESPs and email builders adds hidden markup that accumulates fast
• Repeated modules and inline CSS bloat file size, especially in longer templates
• Personalization logic and conditional content can quietly push emails over the limit

The practical fix is designing with a 75KB target, leaving a buffer for the code that gets added during send. Industry analysis of over 6,000 emails found that roughly 1 in 18 exceeded the clipping threshold, a number that sounds small until you consider enterprise send volumes. Shortening tracked URLs, minifying HTML, and auditing template code regularly to remove unused modules all help keep file size under control.

Dark mode rendering across email clients

Dark mode has moved from niche preference to default behavior. Roughly 82% of smartphone users have dark mode enabled on their devices, and industry estimates place 35% to 50% of email opens in dark mode environments. For audiences that skew heavily toward Apple iOS Mail, that number can reach 60% to 70%.

The rendering challenge is that email clients implement dark mode differently: some invert colors automatically, some partially adjust specific elements, and others leave emails untouched entirely. There is no single standard, which means an email designed for light mode might look fine in Apple Mail's dark mode but produce invisible text on a dark background in Outlook's version.

The most common dark mode failures are predictable: logos with non-transparent backgrounds get white boxes around them, light text on light backgrounds becomes illegible, and brand colors shift in ways that break visual hierarchy.

What makes dark mode particularly difficult is the gap between user adoption and email optimization. Despite the majority of mobile users having dark mode enabled, only about 11% of emails contained dark mode-specific code as of late 2023. That gap is wider in B2B, where roughly 39% of brands optimize for dark mode compared to 48% on the B2C side.

Meyer's advice is pragmatic: "Dark mode's not niche anymore. If your audience skews mobile, your exposure to it is likely even higher. Time to add it to the checklist."

Accessibility failures most teams don't catch

The Email Markup Consortium's 2025 accessibility report analyzed 443,585 HTML emails and found that 99.89% contained accessibility issues categorized as "Serious" or "Critical." Fewer than 0.01% passed all automated checks, and those came from just two brands.

That is not a rounding error. Web accessibility has had WCAG standards and enforcement for over a decade. Email accessibility has not had the same attention, and it shows.

The most common failures include missing language direction attributes (present in 98% of emails tested), absent or meaningless alt text, broken heading hierarchy, and insufficient color contrast. These are not edge cases but the default state of enterprise email, built into the templates and processes that most teams have used for years without auditing.

Part of the problem is tooling, and this is worth knowing: only one email client (SFR Mail) supports all HTML and CSS accessibility features. Apple Mail and Samsung Email come close, but most clients impose their own limitations that make full accessibility difficult even when the code is correct.

The other part is process. Fewer than half of companies incorporate basic accessibility measures like alt text in their email production workflow. Meyer identifies the blind spot: "Email accessibility shouldn't be optional, but it's wildly under-prioritized. Not on purpose. It's just not something most teams even realize they should be checking."

Building a rendering QA process that catches issues before send

So what actually works? The teams that consistently avoid rendering failures share a pattern: they test against their actual audience data rather than trying to cover every possible email client.

Meyer's framework starts with audience analysis. "Pull 90 days of open data by email client. You'll be able to identify which clients your audience uses, how much each one matters, and how your audience splits across desktop vs. mobile." If 65% of your audience opens in iOS Mail and Gmail, those get testing priority. But if you are sending to enterprise accounts where Outlook dominates, your testing matrix looks very different.

From there, weight testing effort by business risk. A revenue-driving promotional campaign warrants deeper testing than an internal newsletter. A transactional email with legal compliance requirements needs the full checklist.

A rendering QA process that goes beyond the basics includes:

"Instead of obsessing over all 60 inbox previews just because a platform shows you 60," Meyer advises, "focus on what actually matters: your email audience."

The rendering problems in email are not going away. Outlook will keep using the Word engine in enterprise environments for years, Gmail will keep clipping at 102KB, and dark mode adoption will keep climbing. The teams that treat rendering QA as a systematic process rather than a pre-send spot check will be the ones whose emails actually perform the way they were designed to.

See how Knak handles email rendering across clients.