MJML: What It Is and How It Improves Email Development

MJML, short for Mailjet Markup Language, is an open-source markup language for building responsive email templates. Instead of hand-coding email HTML and its many table-based, client-specific workarounds, developers write semantic MJML components—such as sections, columns, text, images, and buttons—then compile them into email-ready HTML.

What is the MJML email framework?

The MJML email framework is a development layer that sits before HTML in an email production workflow. It is not an email service provider, a sending protocol, or an inbox-placement tool. It is a language and rendering engine designed to make the creation of responsive email markup more predictable.

That distinction matters. Email clients do not receive or render MJML. They receive the HTML that MJML generates. A recipient opening a campaign in Gmail, Outlook, Apple Mail, or another client sees compiled email HTML, including the tables, inline styles, and conditional markup necessary for broad compatibility.

Email HTML differs sharply from ordinary web HTML. Many email clients restrict CSS support, handle margins inconsistently, remove certain styles, block external resources, or rely on rendering engines with legacy behavior. A layout that is simple in a web page can require nested tables, presentation attributes, inline CSS, and client-specific fallback code in an email.

MJML lets a developer express the intended layout at a higher level. For example, a developer can describe a content area with a section, a column, a heading, body copy, and a call-to-action button. The MJML engine translates that structure into HTML designed for email-client constraints.

This makes MJML particularly useful for:

• Transactional email templates, such as password resets, receipts, shipment updates, and account alerts.
• Marketing campaigns that need a consistent visual system across repeated sends.
• Lifecycle messages, including onboarding, renewal, trial-expiration, and re-engagement emails.
• Product teams that want email templates reviewed, versioned, and deployed alongside application code.
• Agencies and design systems that need reusable email components rather than one-off HTML files.

MJML is often described as a responsive email framework because its standard components abstract much of the layout work needed to create an email that adapts between desktop and narrower mobile screens. That does not eliminate the need for testing, but it reduces the amount of low-level email markup a team needs to write and maintain.

Why MJML exists

The central problem MJML addresses is not simply “writing an email.” It is writing HTML that remains usable across a fragmented ecosystem of inboxes and applications.

A web developer can usually rely on modern layout systems such as flexbox, grid, external stylesheets, JavaScript, and a broadly consistent browser engine. Email developers cannot safely make the same assumptions. Inbox providers and desktop clients differ in what they support, sanitize, ignore, or reinterpret.

A two-column desktop layout is a useful example. On a website, it may require only a few CSS declarations. In an email, the same design often needs table-based structure, fixed or percentage widths, mobile-specific behavior, carefully placed padding, and fallbacks for clients with limited CSS support. A button may need more than an anchor tag and background color if the sender wants it to look reliable in clients with unusual rendering behavior.

MJML does not make all email-client differences disappear. Instead, it turns common layout patterns into components with familiar, declarative names. A developer can focus on the message structure and content while the compiler handles much of the repetitive output.

The result is usually easier to reason about than a large HTML file containing deeply nested tables and hundreds of inline style declarations. This can improve development speed, reduce accidental layout regressions, and give code reviewers a clearer representation of the intended email.

Semantic intent versus generated markup

With MJML, the source template expresses intent:

• mj-section represents a horizontal row or content band.
• mj-column represents a column within that row.
• mj-text represents styled text content.
• mj-image represents an image block.
• mj-button represents a call to action.
• mj-divider represents a visual divider.
• mj-spacer represents intentional vertical space.

The compiler then produces the lower-level HTML necessary to approximate that intent in email clients. This separation is valuable because a layout change can often be made in one understandable place rather than by editing repeated table structures throughout a template.

MJML is source code, not the delivered message format

A practical way to think about MJML is similar to a source language that compiles into a delivery format. The .mjml file is the authoring source. The resulting .html file is what a sending platform accepts and what email clients ultimately render.

That means a production workflow must include compilation. A transactional application might compile templates during its build process. A campaign team may compile templates before importing or sending them through an email platform. Some visual editors can also work with MJML as an import or editing format.

How MJML works from template to inbox

An MJML document has a predictable top-level structure. The root mjml element contains an optional mj-head and a required mj-body for visible message content.

The head is where authors commonly place metadata, preview text, fonts, shared styles, and component defaults. The body contains the visible layout. Within the body, sections contain columns, and columns contain content components such as text, images, buttons, dividers, and social links.

Here is a small but realistic example:

<mjml>
  <mj-head>
    <mj-title>Your weekly account summary</mj-title>
    <mj-preview>Your activity, updates, and next steps.</mj-preview>
    <mj-attributes>
      <mj-text font-family="Arial, sans-serif" font-size="16px" line-height="1.5" color="#1f2937" />
      <mj-button background-color="#2563eb" color="#ffffff" border-radius="4px" font-weight="700" />
    </mj-attributes>
  </mj-head>

  <mj-body background-color="#f3f4f6" width="600px">
    <mj-section background-color="#ffffff" padding="24px">
      <mj-column>
        <mj-text font-size="28px" font-weight="700" padding="0 0 12px">
          Your weekly summary
        </mj-text>
        <mj-text padding="0 0 20px">
          You completed 8 tasks this week and have 3 items due next week.
        </mj-text>
        <mj-button href="https://example.com/dashboard" padding="0">
          Open your dashboard
        </mj-button>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>

The source code is compact because it does not show every HTML table, conditional comment, or inline declaration that may be necessary in the final output. When compiled, the resulting HTML is longer than the MJML source and is intended to be delivered as the email body.

The core layout model

The most common MJML layout hierarchy is:

1. mj-body defines the overall email canvas and body-level properties.
2. mj-section creates a row-like horizontal area.
3. mj-column divides a section into one or more columns.
4. Body components, such as mj-text and mj-button, sit inside a column.

By default, MJML divides a section’s available width among its declared columns. A section with two columns generally creates a two-column desktop layout, while the generated email markup is designed to support a responsive presentation on narrower screens.

For example:

<mj-section background-color="#ffffff" padding="16px">
  <mj-column width="50%">
    <mj-image src="https://example.com/product.jpg" alt="Blue travel backpack" />
  </mj-column>
  <mj-column width="50%">
    <mj-text font-size="20px" font-weight="700">Weekend travel bag</mj-text>
    <mj-text>Carry-on friendly, water resistant, and available in three colors.</mj-text>
    <mj-button href="https://example.com/products/travel-bag">View product</mj-button>
  </mj-column>
</mj-section>

This is not a guarantee that every client will display every pixel identically. It is a structured way to begin with email-aware output rather than manually recreating a two-column table layout for every message.

Shared defaults and reusable styles

The mj-attributes component is especially useful for maintaining consistency. It lets a team define default attributes for a component type or create named attribute groups that can be reused with mj-class.

That matters for a design system. Without defaults, changing a body font, link treatment, or button style may mean editing many individual blocks. With a shared definition, teams can centralize common visual decisions and reduce drift between templates.

A simple pattern looks like this:

<mj-head>
  <mj-attributes>
    <mj-text font-family="Arial, sans-serif" font-size="16px" line-height="1.5" color="#222222" />
    <mj-class name="primary-button" background-color="#111827" color="#ffffff" border-radius="6px" font-weight="700" />
  </mj-attributes>
</mj-head>

The class can then be applied to a button:

<mj-button mj-class="primary-button" href="https://example.com/confirm">
  Confirm email address
</mj-button>

MJML syntax and commonly used components

MJML uses XML-like tags and attributes. Correct nesting matters: a component must be placed where the language expects it. For example, visible content components generally belong inside an mj-column, which belongs inside an mj-section.

The standard component set covers many common email patterns. The following components are among the ones developers encounter most often:

The right component is usually the one that most clearly expresses the content’s purpose. Use mj-text for normal text rather than forcing text into a raw HTML block. Use mj-button for a prominent call to action rather than trying to construct a button from arbitrary markup. Reserve mj-raw for cases where the standard component library does not cover a necessary pattern.

Head components that affect the recipient experience

Several head-level components are operationally important even though recipients do not see them as page content.

mj-title sets the document title in the generated HTML. mj-preview supplies preview text, which can appear beside or beneath the subject line in inbox interfaces. Preview text should support the subject line rather than repeat it. If a subject says “Your order has shipped,” preview text might say “Track package 84721 and see the expected delivery date.”

mj-font can declare a web font for clients that support it, but every production email should include sensible fallback fonts. A distinctive web font may not load in many inboxes, so the design must remain readable with the fallback stack.

mj-style can add CSS, while mj-attributes is generally better for component-level defaults. Both should be used conservatively. Email-client support varies, and styles that work in one inbox may be ignored or altered in another.

Personalization is separate from MJML

MJML controls presentation and layout. Recipient-specific content is a different layer.

A sender may add template variables or merge syntax supported by its sending platform, application framework, or templating engine. For example, an application could replace a placeholder with the recipient’s first name, order number, or account URL before the email is sent. The exact variable syntax is platform-specific, so it should be confirmed in the documentation for the sender’s chosen email provider.

Keep this separation clear: MJML describes the email’s structure; the application or sending platform supplies personalized data; the final compiled HTML is sent through SMTP or an email API.

Why MJML matters for deliverability

MJML is not a deliverability metric, and using it does not directly cause inbox placement. Mailbox providers do not reward an email merely because it was authored in MJML. They evaluate the delivered message and the sender’s overall practices, including authentication, reputation, complaint patterns, engagement, list quality, and message characteristics.

Still, template quality can influence deliverability indirectly. A broken, inaccessible, confusing, or untrustworthy email can reduce engagement, increase complaints, produce more unsubscribe requests, or cause recipients to ignore future messages. Those outcomes can weaken campaign performance and, over time, contribute to worse sending results.

MJML can help by making it easier to produce consistently structured, responsive templates. The emphasis is on reducing avoidable rendering problems, not bypassing mailbox filtering.

Indirect deliverability benefits

A well-maintained MJML workflow can support deliverability and performance in several indirect ways:

• More reliable rendering: Clear layouts and dependable call-to-action placement reduce the risk that a recipient sees a broken or unusable message.
• Better mobile usability: A significant share of email is opened on small screens. Responsive structure can make essential content easier to read and act on.
• Faster iteration: Teams can correct broken links, improve hierarchy, or simplify a design without repeatedly editing fragile table markup.
• Stronger consistency: Shared attributes and components help keep transactional and campaign templates aligned with approved brand and accessibility standards.
• Cleaner review processes: Source templates are easier to inspect for missing alt text, unclear links, placeholder copy, and unintentional content changes.

These are practical advantages, not filtering guarantees. A beautifully rendered email can still land in spam if it is sent to poor-quality addresses, lacks proper authentication, generates complaints, or comes from a sender with weak reputation.

Deliverability fundamentals MJML cannot replace

Even the best MJML template needs a sound sending program. Priorities include authenticated sending domains, consent-based list building, accurate recipient data, sensible sending cadence, functional unsubscribe handling for promotional mail, and monitoring of delivery events.

For transactional senders, content must also match the recipient’s expected action. A password reset, invoice, or login alert should be recognizable, timely, and easy to verify. Adding unrelated promotional content to a security-related message can confuse recipients and increase suspicion.

MJML helps with the message’s construction. It does not replace domain authentication, suppression handling, sending policies, or recipient consent management.

MJML and campaign performance

Campaign performance is broader than deliverability. It includes whether recipients notice, understand, trust, and act on an email after it reaches the inbox.

MJML can improve the operational side of performance work because it makes template experiments more controlled. A team can retain the same layout framework while testing one variable at a time: subject line, offer positioning, call-to-action copy, hero image, content order, or button color. When the markup is component-based, a small intended change is less likely to create unrelated visual damage elsewhere in the template.

Responsive design affects action, not just appearance

A desktop-first email that squeezes poorly onto a phone may technically render but still fail to perform. Small text, tightly packed links, large images that push the primary action below the fold, and side-by-side columns that become hard to scan can all introduce friction.

A good responsive layout keeps the primary message understandable without requiring recipients to pinch, zoom, hunt for a button, or load every image. The first screen should make the email’s purpose obvious. For a transactional email, that often means a short explanation, the relevant record or action, and one clear destination.

For a campaign, it can mean a recognizable brand signal, a concise offer, and a primary call to action near the top. MJML helps authors implement those patterns, but it does not decide the strategy. Content hierarchy, audience segmentation, and offer relevance remain human decisions.

Accessibility is performance work

Accessibility improvements often improve usability for every recipient. Clear hierarchy, meaningful link text, adequate contrast, descriptive image alternatives, and readable font sizes make an email more understandable in assistive technology and in constrained viewing conditions.

For images, use meaningful alt text when the image communicates information. If an image is purely decorative, an empty alternative may be more appropriate than repetitive description. Avoid making an image the only place where critical copy or a call to action appears, because images may be blocked by default or unavailable to some recipients.

For buttons, use action-specific labels such as “View invoice,” “Reset password,” or “Track shipment.” A vague label like “Click here” forces a recipient to infer the destination and provides weak context when read by assistive technology.

Is MJML a metric that can be measured or calculated?

No. MJML is not a rate, score, or deliverability measurement, so there is no MJML formula to calculate.

It is a markup language and compilation workflow. You can measure outcomes associated with an MJML-based email program, but those outcomes are separate metrics. Examples include delivered messages, hard bounces, soft bounces, complaint rate, unsubscribe rate, click rate, conversion rate, rendering defects found in testing, and template build time.

A team can also establish internal engineering measures around its MJML workflow. For example:

• Number of reusable components shared across templates.
• Number of rendering defects discovered before launch.
• Time required to create a new transactional template.
• Percentage of templates that include preview text, image alternatives, and tested primary links.
• Number of templates using current design-system defaults rather than copied legacy markup.

These measures can reveal whether the workflow is improving quality and speed. They should not be confused with an inbox-placement score.

A practical numeric example: measuring template QA improvement

Although MJML itself is not a metric, a team can measure whether its template process reduces errors.

Suppose a team reviews 20 campaign and transactional templates before adopting shared MJML components. Test sends reveal 14 visual or functional issues: missing image alternatives, broken spacing on mobile, unstyled links, inaccessible button labels, and inconsistent padding.

After creating shared defaults for text, buttons, and section padding, the team reviews another 20 templates. This time it finds 5 issues.

The reduction in issues is:

(14 - 5) / 14 × 100 = 64.3%

That does not prove MJML alone caused the improvement; better review discipline and a more mature design system may also contribute. But it gives the team a concrete way to evaluate whether a component-based workflow is reducing avoidable template defects.

Common MJML problems and their causes

When an MJML email does not render as expected, the cause may be in the source structure, the compiler output, the email client, the sending platform, or the data inserted into the template.

Invalid nesting or unsupported placement

A common source error is placing a component outside the structure where MJML expects it. For example, content intended for a column may be placed directly inside a body or section. The MJML validator can identify many structural issues before a send.

Fix this by returning to the hierarchy: body, section, column, then content component. Keep the layout simple before adding special cases.

Overusing raw HTML

mj-raw can be useful for specialized content, but it also bypasses much of the abstraction that makes MJML safer to maintain. Raw blocks can introduce client-specific failures, CSS conflicts, unclosed tags, or unpredictable layout behavior.

Use raw HTML only when a standard component cannot satisfy a genuine requirement. Document why it is needed, isolate it to a small area, and test it more aggressively than ordinary components.

Treating desktop preview as final QA

A browser preview is useful, but it is not the same as validating a delivered email in real inboxes. Email clients may alter CSS, rewrite links, block images, show dark-mode changes, or display layout differences that are not visible in a local preview.

Fix this by sending tests to a representative set of inboxes and devices. At minimum, inspect major client categories relevant to the audience, including a webmail interface, a mobile client, and the Outlook environments used by important business recipients.

Missing fallback content

A design can look complete while depending too heavily on images, custom fonts, or a single layout behavior. If images are blocked, the email should still communicate its essential message. If a custom font fails, the fallback should preserve readability. If a two-column layout changes on mobile, the reading order should still make sense.

Fix this by designing for failure modes. Put key copy in live text, write useful image alternatives, choose practical fallback fonts, and make the primary action clear without relying on visual decoration.

Uncontrolled content insertion

Personalized fields can create layout problems when values are unexpectedly long, empty, malformed, or contain characters that require escaping. A first name might be 30 characters long; a product title may contain punctuation; a user-generated value may include content unsuitable for direct insertion.

Fix this with data validation, sensible fallbacks, and realistic test records. Test with long names, missing values, non-English characters, long product names, and unusual URLs before launching.

How to improve an MJML email workflow

The strongest MJML implementations treat templates as production assets rather than disposable campaign files. They use conventions, reviews, version control, and repeatable tests.

Build a small component system first

Start with a limited set of reusable patterns rather than creating every email from scratch. A practical initial library might include:

1. A standard outer wrapper and content width.
2. A header with logo treatment and safe spacing.
3. A primary text style and a smaller supporting text style.
4. A primary button and secondary text-link treatment.
5. A product or content card pattern.
6. A footer with required business details and preference or unsubscribe links where applicable.

This approach creates consistency without forcing every message into the same rigid design. Teams can add specialized components later when repeated needs emerge.

Keep the source readable

Readable MJML is easier to maintain, review, and troubleshoot. Use consistent indentation, keep component attributes organized, avoid unnecessary nesting, and give templates descriptive filenames. Separate long content blocks from presentation rules where the toolchain allows it.

Do not optimize source brevity at the expense of clarity. A few extra lines in the MJML file are usually preferable to a clever but opaque structure that another developer cannot safely update.

Compile and validate before deployment

The MJML command-line interface can render a source file into HTML. A basic command is:

mjml input.mjml -o output.html

Treat compilation as a build step, not a manual afterthought. In a mature workflow, a change to an MJML template can trigger syntax validation, HTML generation, link checks, snapshot comparison, and test sends before the template reaches production.

For teams sending through an API, the compiled output is the asset that should be passed into the sending workflow. Review the email API reference and setup guides when connecting rendered template HTML to an application’s transactional sending pipeline.

Test the actual message

Testing should cover more than whether the template compiles. Before sending at scale, check:

• Subject line and preview text together in an inbox.
• The visible sender name and sender address.
• Primary and secondary links, including UTM or tracking parameters.
• Rendering with images enabled and disabled.
• Mobile readability, tap targets, and content order.
• Dark-mode behavior where it is relevant to the audience.
• Personalization with normal, missing, and unusually long values.
• Footer content, preference links, and unsubscribe behavior for promotional email.

Testing is also the point to inspect the final HTML rather than assuming source intent survived every system in the pipeline. A sending platform, link-tracking service, template processor, or content security layer may alter parts of the message after MJML compilation.

MJML compared with hand-coded HTML and visual editors

MJML is one option in a broader email-production spectrum.

Hand-coded HTML offers maximum control and can be appropriate for teams with deep email expertise, unusual legacy requirements, or templates that depend on markup beyond a framework’s components. The tradeoff is maintenance cost. Email HTML is verbose, and a simple visual change can touch many low-level elements.

Visual drag-and-drop editors are useful for teams that need fast assembly and collaboration without code. They can work well for routine campaigns, especially where non-developers own production. The tradeoff is that highly customized layouts, code review, source control, and component reuse may be harder depending on the editor.

MJML sits between these approaches. It gives developers a source format that is more structured and expressive than raw email HTML while still compiling to standard HTML for delivery. It can also coexist with visual workflows; some platforms support MJML import or use it as an editable template representation.

The right choice depends on the team. If marketers need full self-service and designs are standard, a visual editor may be sufficient. If developers own transactional templates and want code-level review, MJML is often a practical fit. If a project needs highly specialized email behavior, raw HTML may still be necessary in selected places.

A sensible production checklist for MJML emails

Before publishing or deploying an MJML template, use a repeatable checklist:

• Confirm the template compiles without validation errors.
• Check that content components are nested correctly inside sections and columns.
• Verify all images have appropriate alt text or are intentionally decorative.
• Confirm every button and text link points to the correct destination.
• Use meaningful call-to-action copy rather than generic language.
• Check preview text and subject line as a pair.
• Test with missing and long personalization values.
• Inspect the message on mobile as well as desktop.
• Ensure critical information is present as live text, not only inside images.
• Confirm the final sending domain and authentication setup are appropriate for the message stream.
• Send seed tests to real inboxes before a major campaign or template change.
• Keep the compiled HTML and MJML source under version control when possible.

The checklist is deliberately broader than syntax. A valid MJML document can still be an ineffective email if the message is unclear, the link is wrong, the offer is irrelevant, or the footer does not meet the sender’s obligations.

Conclusion

MJML is a markup language for creating responsive email templates more efficiently than hand-coding every detail of email HTML. Its component model gives teams a clearer way to define sections, columns, copy, images, buttons, defaults, and reusable patterns, while its compiler generates HTML intended for the realities of email clients.

It is not a deliverability shortcut and not a metric. Sender reputation, authentication, consent, list quality, recipient expectations, and campaign relevance remain essential. But MJML can reduce rendering errors, support a more disciplined template workflow, improve maintainability, and help teams spend more time on message quality instead of repetitive markup.

For transactional and campaign email programs alike, the practical goal is simple: create messages that are clear, accessible, trustworthy, responsive, and thoroughly tested before they reach recipients.

FAQ

What does MJML stand for?

MJML stands for Mailjet Markup Language. It is an open-source markup language created to simplify the development of responsive email templates.

Does MJML send emails?

No. MJML creates source templates and compiles them into HTML. You still need an email delivery method, such as SMTP or an email API, to send the compiled HTML to recipients.

Does MJML improve email deliverability?

Not directly. Mailbox providers do not use MJML authorship as an inbox-placement signal. MJML can indirectly help by making templates more reliable, readable, and maintainable, but authentication, reputation, consent, and list quality are the primary deliverability factors.

Can recipients see MJML in an email?

No. Recipients receive the compiled HTML output, not the MJML source. Their email client renders that HTML.

Should every email be built with MJML?

Not necessarily. MJML is a strong option for teams that want a code-based, reusable, responsive email workflow. A visual editor may be better for nontechnical campaign teams, while hand-coded HTML may be appropriate for unusually specialized templates or legacy requirements.