Why Emails Break Even When the Code is Clean

The Rendering Reality Check

Ever had that perfect email campaign turn into a design disaster?

Picture this scenario: your marketing operations team spends weeks perfecting an email campaign. The design looks perfect in your preview, and the code validates flawlessly. You hit send to millions of subscribers, confident everything is under control… then the angry screenshots start rolling in. Your CEO's email looks like abstract art in Outlook, the sales team can't click the CTA button on their phones, and half your European offices are dealing with broken layouts in dark mode.

Welcome to the frustrating reality of email rendering in 2025.

The illusion of control

Almost every email marketer starts off with the same assumption that logically, if the code works in one email client, it should work everywhere. But we quickly learn that that just isn’t the case.

The same asset that sparkles in Gmail might look completely broken in Outlook. A button that works perfectly on desktop Apple Mail could vanish entirely on Android. This isn't a bug. It's the fundamental nature of email technology.

Because of decades of competing standards and corporate politics, every email client has its own way of rendering HTML and CSS.

And unfortunately, marketers can’t just pick one or two major clients to optimize for, and ignore the rest… Apple Mail commands 49.8% to 57% of email opens, while Gmail holds 26.7% to 30%, and Outlook maintains 7% to 8.1% of market share (So no, we can’t just say ‘Well no one uses Outlook, let’s move on.')

The reality is that as much as we wish we could control every aspect of how something may look in any inbox, there are elements beyond our influence that we simply have to work around and test.

Rendering impacts more than just looks

When emails break, the damage extends far beyond aesthetics... they impact the recipient's trust in the brand as a whole. About 42.3% of users immediately delete emails not optimized for their device. And for enterprises sending millions of emails, that loss of trust and communication can result in massive losses of potential revenue

The complexity of dealing with all these rendering issues can slow down your marketing team as well, and create a bottleneck when you're trying to get campaigns out quickly.

Consider what happens when your carefully designed email fails. That broken CTA button isn't just a design flaw, it's a conversion killer. The illegible text in dark mode doesn't just look unprofessional, it also damages your external brand equity.

Additionally, when internal stakeholders receive broken emails from their own company, confidence in marketing operations plummets.

The marketing operations challenge

As a marketing operations professional, you're responsible for consistency across global teams, multiple email service providers, and complex legal workflows. You manage templates used by dozens of marketers across different regions, each with unique requirements: from legal teams that need specific disclaimers, to regional teams that require localization, to brand stakeholders who expect adherence to guidelines.

Meanwhile, you're juggling multiple ESPs, each with its own method of handling and sometimes altering the code that you feed into it. They might rewrite certain parts of the HTML, or they might handle CSS differently, which can sometimes lead to unexpected issues once the email is actually delivered. By the time your email reaches the inbox, it's been processed, parsed, and potentially mangled by multiple systems.

Understanding the email rendering stack

Rendering engines explained

The root of email rendering chaos lies in the engines themselves. Outlook 2007 through 2019 uses Microsoft Word's rendering engine. Yes, Word. The same program you use for documents is interpreting your HTML. This explains why Outlook struggles with background images, positioning, and modern CSS features.

Gmail runs on a modified WebKit engine with its own restrictions. It clips emails larger than 102KB(kilabites) and strips certain CSS properties.

Apple Mail uses WebKit, offering the strongest CSS3 support, but introduces its own challenges with automatic dark mode adaptation and caching issues.

Yahoo Mail, commanding a very small amount of market share, uses a custom rendering engine with its own peculiarities.

Each engine interprets HTML and CSS through its own lens, creating a fragmented landscape where consistency becomes nearly impossible.

The enterprise matrix

The complexity multiplies when you factor in the enterprise environment. Your emails must work across:

• Multiple devices (desktop, tablet, smartphone)
• Various operating systems (Windows, macOS, iOS, Android)
• Different display settings (standard, high DPI(dots per inch), retina)
• Numerous email client versions (some companies still run Outlook 2007)

Each combination creates a unique rendering scenario.

Dark mode and accessibility

34% to 35% of users now open emails in dark mode, yet only 11% of marketers always design for it. Dark mode doesn't just change how the inbox appears; it fundamentally changes how emails themselves render.

White logos can disappear against dark backgrounds, while light gray text often becomes unreadable. Carefully chosen brand colors may invert or shift in unexpected ways, and what looks professional in light mode can quickly turn into an accessibility nightmare once dark mode is enabled.

Dark mode isn’t the only accessibility lever that gets impacted by inbox rendering, though. Font rendering varies between clients, affecting readability, and color contrast that meets WCAG standards in one client might fail in another. Missing alt text also becomes glaringly obvious when images fail to load.

Every email client handles dark mode, colors, and fonts in its own way, so designing for accessibility means planning for a wide range of unpredictable environments.

What breaks and how to build around it

HTML and CSS limitations

Email HTML is not evolving with time. While modern web development has embraced flexbox, CSS grid, and semantic structure, email design is stuck using tables and inline styles.

Here’s what routinely breaks: Flexbox and CSS Grid aren’t supported by most email clients, making them unreliable choices for layout. Float behaves inconsistently, especially in Outlook. Positioning elements with absolute or relative values rarely works as expected. And media queries? While some clients honor them, many strip them out entirely, leading to unpredictable results.

Even the basics behave unpredictably, with margins collapsing in different ways depending on the client and padding being calculated inconsistently from one platform to another.

For years, the only reliable way to get emails rendering correctly was to build like it’s 1999: use tables for layout, inline every style, and favor repetition over elegance.

But let’s be clear, we’re not saying you should go back to 1999.

We’re saying that was the only option. Until now.

This is exactly the problem Knak was built to solve. Instead of wasting time coding around limitations and testing 50 versions of the same email, Knak takes care of rendering quirks behind the scenes. You get modern, creative freedom paired with reliable, cross-client compatibility. No guesswork, no dev tickets, and no Outlook surprises.

Because emails shouldn’t break just because someone opens them in a different client.

Outlook specific pain points

Outlook deserves special attention for its unique ability to destroy layouts. The Word rendering engine brings peculiar limitations:

• Background images in tables? Not supported
• Animated GIFs? They freeze on the first frame
• Bullet points? They might render as squares, circles, or not at all
• DPI scaling on high-resolution screens? Prepare for layout explosions
• Rounded corners

These aren't rare glitches or one-off bugs. They're daily realities for the millions of people still using Outlook in corporate environments.

The ESP curveball

Your email service provider (ESP) adds another layer of complexity. ESPs inject their own code for tracking, personalization, and dynamic content. This injection can break your carefully crafted layouts.

Salesforce Marketing Cloud wraps your links in redirect URLs. Adobe Campaign may rewrite your HTML tags. Eloqua's locked templates protect against some issues but limit flexibility. Each platform has its own method of handling dynamic content, and each method introduces potential breaking points.

Personalization only compounds the problem, since dynamic content blocks can introduce unclosed tags, and variable-length text has the potential to break layouts that were designed with specific content in mind. Conditional logic might work flawlessly for one segment, yet fail completely for another, creating inconsistencies that are difficult to catch before send.

Testing and QA from chaos to process

Cross client testing is non negotiable

Testing in Gmail alone isn’t enough to ensure your email will render correctly across all platforms. It only gives you a partial view and leaves you blind to how your message performs in other major clients. Professional email testing requires previewing across 100+ client and device combinations

The bare minimum testing matrix includes:

• Desktop: Outlook (multiple versions), Gmail web, Apple Mail, Yahoo
• Mobile: iOS Mail, Gmail app (iOS/Android), Outlook app
• Web clients: Gmail, Outlook.com, Yahoo Mail
• Dark mode variations of each

This creates dozens of test scenarios for a single email. Miss even one, and you risk alienating a significant portion of your audience.

Enterprise grade QA

Enterprise email QA transcends simple rendering checks.

Your process must validate:

• Rendering across all major clients and devices
• Dark mode and light mode variations
• Link functionality and tracking
• Dynamic content and personalization logic
• Accessibility compliance
• Regional variations and translations
• Legal and compliance requirements

Each step requires formal documentation and sign-off, with marketing requesting content approval and design needing creative review. Legal teams must verify compliance, regional teams are responsible for validating translations, and IT security may require link scanning. The QA process quickly becomes a complex orchestration that involves multiple stakeholders, each with their own priorities and checkpoints.

Tool stack recommendations

Professional email testing demands professional tools. Platforms like Litmus Enterprise and Email on Acid offer detailed previews, accessibility audits, and strong collaboration features. These are essential for enterprise teams managing complex campaigns.

But tools alone won’t save you.

Even with the right platforms, reliable email rendering depends on well-defined processes. You need more than just software, you need a system. That includes documented testing protocols, clear ownership across teams, and automation wherever possible.

Build a checklist that matches your rendering priorities. Document known client quirks and brand guidelines. Set up clear escalation paths for when issues arise. And always test twice(once before stakeholder review, and again after final approvals).

Of course, all of this becomes much easier with Knak.

Instead of juggling third-party tools, fragile templates, and last-minute workarounds, Knak builds rendering best practices (and testing!) right into the platform. It is designed with enterprise standards in mind, so your emails render correctly the first time, every time.

Scalable Email Templates for Enterprise Teams

Build modular not monolithic

Enterprise email templates must withstand constant change. Marketing teams will swap images, copy will expand or contract, and regional teams will often add unexpected content. Your templates need to be flexible enough to handle these shifts without breaking.

Modular design principles save sanity. Create independent components that stack together: headers, hero sections, content blocks, CTAs, footers. Each module should work independently and in combination with others. When marketing needs a new layout, you assemble existing modules rather than building from scratch. Knak makes modular design scalable by giving teams a shared system of reusable, brand-safe components they can quickly configure to meet campaign needs.

This approach requires upfront investment but pays dividends in reduced maintenance and fewer rendering issues. A well-designed modular system can reduce email production time while improving consistency.

Design for failure

Every email component should anticipate failure. Buttons must work without images and layouts should survive without CSS. Content must remain readable even when everything else breaks.

Bulletproof buttons use HTML text over solid backgrounds rather than images. They survive when images are blocked and remain clickable even in the most restrictive clients. Fallback fonts prevent rendering disasters when custom fonts fail. Alt text provides context when images don't load.

Design decisions should prioritize function over form, because that gorgeous custom font might not render and the innovative layout could easily collapse. But if the message comes through clearly and the CTA works as intended, the email still succeeds.

Rendering resilience now and what's next

No-code to the rescue

The rise of no-code email builders, like Knak, represents a fundamental shift in email creation. These platforms abstract away the rendering complexities, automatically generating client-specific code optimizations.

For enterprise marketing ops, no-code and modular tools bring serious advantages. They lock in consistency by using reusable components, reduce production time from weeks to hours, and eliminate the need for marketers to know HTML. Most importantly, they handle rendering quirks behind the scenes so emails display correctly across every client, every time.

Marketing operations challenge

Your role extends beyond fixing broken emails. You're responsible for building sustainable systems that support email production at scale. This includes creating documentation that holds up through team changes, developing processes that can grow alongside the business, and establishing governance that helps prevent problems before they happen.

Success depends on a mix of technical expertise and organizational skill. You need to understand why emails break, while also managing stakeholders who may not. You’re expected to build reliable systems that support rapid change without sacrificing quality.

The companies that excel at email rendering are not just working with better code. They also have stronger processes, clearer documentation, and more effective governance. They approach email production as a critical business system rather than treating it as an afterthought.

Build for resilience not perfection

Email rendering will never be perfect, with too many variables, too many legacy systems, and too much technical debt spread across the entire email ecosystem. But perfection isn’t the goal. Resilience is!

Build systems that bend without breaking. Create templates (or even better: modules) that survive real-world usage. Establish processes that catch problems before they reach inboxes.

The emails that succeed aren't necessarily the most beautiful. They're the ones that work everywhere, for everyone, every time. In enterprise marketing operations, consistency beats creativity. Reliability matters more than innovation. And testing, always testing, remains your best defense against the chaos of email rendering.

Your next email campaign will face the same rendering challenges as your last one. But with the right processes and tools in place, those challenges become manageable rather than maddening. If you build for the reality of email rendering your emails will reach their destination intact.