Definition
MJML (Mailjet Markup Language) is an open-source email framework designed to simplify responsive email development. It provides a set of semantic components that compile to production-ready HTML email code. Instead of hand-coding complex table structures, media queries, and VML fallbacks, developers write MJML components like <mj-section>, <mj-column>, <mj-button>, and <mj-image>, which the MJML compiler translates into the necessary HTML, CSS, and conditional comments.
The MJML compile process transforms high-level components into the intricate table-based layouts required for email rendering. An <mj-section> becomes a full-width table, <mj-column> becomes a nested table cell with appropriate widths, and <mj-button> generates the bulletproof button code including VML for Outlook. The compiler handles media queries, inline CSS, and fallbacks automatically, allowing developers to work at a higher level of abstraction.
MJML is widely adopted in the email industry, with support from major email service providers including Mailjet, Sendinblue, and others. The framework is maintained by Mailjet (now part of Sinch) and has a large community contributing plugins and components. However, MJML has limitations: it generates verbose HTML compared to hand-coded email, it requires understanding the MJML component model in addition to email fundamentals, and complex custom layouts may require falling back to raw HTML within MJML files.
Best Practices
Learn the fundamentals of HTML email before adopting MJML. MJML abstracts complexity but does not eliminate it. Understanding tables, inline CSS, and conditional comments helps you debug issues that arise from the compiled output.
Keep MJML components simple and avoid deep nesting. MJML's compiled output grows exponentially with nesting depth, producing HTML that can be 3–5 times larger than hand-coded equivalents. Flatter component structures produce cleaner output.
Use MJML's built-in responsive behaviour rather than custom media queries. MJML's <mj-column> components handle mobile stacking automatically. Adding custom media queries increases complexity and reduces the benefit of using the framework.
Extend MJML with custom components for reusable patterns. If you use the same header, footer, or product card across campaigns, create an MJML component to maintain consistency and reduce duplication.
Validate compiled output in email testing tools. MJML handles many cross-client issues, but the compiled HTML should still be tested in Litmus or Email on Acid to verify Gmail, Outlook, and Apple Mail rendering.
Related Glossary Terms
CSS in Email
The subset of CSS properties supported across email clients, dominated by inline styles and constrained by Gmail and Outlook limitations.
Email Bulletproof Button
A cross-client email button technique using VML for Outlook and standard HTML for all other clients, ensuring consistent appearance and clickability.
Email Ghost Table
A responsive email technique using hidden tables to conditionally show or hide content for mobile and desktop layouts across all email clients.
Email Hybrid Coding
A responsive email development approach combining table-based layouts for Outlook with div-based fluid techniques for modern clients.
Email Progressive Enhancement
An email design strategy that builds a functional base layout for all clients then layers enhanced features for more capable rendering engines.
Frequently Asked Questions
Yes. MJML abstracts the tedious parts of email coding but you still need to understand email constraints — table-based layouts, inline CSS, image blocking, conditional comments — to diagnose and fix issues in the compiled output.
They serve different purposes. MJML is a framework that compiles to email HTML. Handlebars is a templating language for injecting dynamic content. Many teams use both: MJML for the email structure and Handlebars (or similar) for dynamic data within MJML components.
MJML does not generate dark mode CSS automatically. You must add custom dark mode styles using MJML's `
MJML generates code designed for broad compatibility, including Outlook via VML bulletproof buttons. However, no framework guarantees perfection in every client. Testing the compiled output in Gmail, Outlook, and Apple Mail is essential.
MJML's compiled HTML is typically 30–80% larger than an equivalent hand-coded email. The trade-off is development speed: MJML can reduce email development time by 50–70% for standard layouts.