Email Template Builder

Definition

An email template builder is a tool or framework used to create HTML email templates that render consistently across the diverse landscape of email clients, webmail interfaces, and mobile devices. Unlike web development where modern CSS is broadly supported, email HTML development requires working within the constraints of outdated rendering engines such as Outlook's Word-based HTML renderer and Gmail's stripped CSS support. Template builders range from visual drag-and-drop editors integrated into ESPs — such as Mailchimp's Creative Assistant, Klaviyo's drag-and-drop editor, and Campaign Monitor's builder — to code-based frameworks like MJML, Foundation for Emails, and custom-coded templates using inline CSS. According to a 2024 Litmus Email Design Survey, 47 per cent of marketers use drag-and-drop builders, 34 per cent use coded templates with frameworks, and 19 per cent build custom-coded templates from scratch.

MJML (Mailjet Markup Language) has emerged as the leading responsive email framework, used by an estimated 25 per cent of email designers according to 2025 industry surveys. MJML abstracts the complexities of email HTML by providing a set of responsive components — such as sections, columns, buttons, and images — that compile to tested, compatible HTML for all major email clients. The open-source framework reduces template development time by 50 to 70 per cent compared to hand-coding, while producing consistently better rendering results across Outlook, Gmail, and Apple Mail. Other frameworks include Foundation for Emails (Zurb), which provides a 12-column responsive grid system similar to web frameworks, and Parcel, which offers a code editor with real-time email preview across Litmus's device testing infrastructure.

Template compatibility testing is essential because email client market share varies constantly and new rendering quirks are introduced with each client update. As of 2025, the most common email clients by market share are Apple Mail (approximately 45 per cent of opens), Gmail webmail and app (28 per cent), Microsoft Outlook desktop and mobile (12 per cent), and Yahoo Mail (4 per cent), though these shares vary significantly by audience demographics. Each rendering engine supports different subsets of HTML and CSS — for example, Outlook for Windows uses Microsoft Word's rendering engine which does not support CSS background images, web fonts, or rounded corners. Template builders and testing services such as Litmus and Email on Acid provide previews across 100 to 150 email client and device combinations, identifying rendering issues before deployment.

Best Practices

Choose a template builder approach based on your team's technical capability and production volume. Marketing teams with limited HTML expertise should use drag-and-drop builders with modular templates that enforce brand constraints while allowing campaign-specific customisation. Teams producing high volumes of unique campaigns should invest in MJML or a similar framework combined with a component library that can be assembled into templates programmatically.

Build a modular template component library rather than designing each campaign from scratch. Create reusable components for headers, footers, navigation bars, content sections (image-left, image-right, full-width text), CTA buttons, product cards, social links, and legal disclaimers. Components should be tested individually across all target email clients and then assembled through the template builder into campaign-specific layouts. Component-based development reduces production time by 40 to 60 per cent after the initial library build.

Test every template across the email clients that represent at least 90 per cent of your audience's open volume before deployment. Use a testing service such as Litmus or Email on Acid to render previews — do not rely on sending test emails to a small set of personal accounts. Pay particular attention to Outlook for Windows (which uses Word rendering), Gmail (which strips external CSS and limits CSS class support), and the default mail app on the latest iOS and Android versions.

Implement a template versioning system that tracks changes to your templates over time, including the MJML or HTML source, the design assets, and the testing results for each version. Use version control practices — commit messages describing the change, tags for production releases, and a changelog documenting when templates were modified and why. Template versioning is essential for troubleshooting rendering issues that emerge after client updates and for rolling back problematic changes.

Monitor template performance analytics beyond rendering — track which templates generate the highest click-through rates, conversion rates, and revenue per email. A/B test template variations on your most-sent campaigns to identify design patterns that drive engagement. Template performance data should inform iterative improvements to your component library and design system, creating a feedback loop between template design and campaign results.